I wanted to put together a little something comparing the changes that each of the new job enhancements provide versus their originals.

So first of all, what is a job enhancement? Job enhancements are very similar to Job , but they are non-unique - so they can be shared by multiple units (ex: both Vettel and Fraise are Holy Cavalier [Isaiyah], whereas Dilga's Holy Cavalier is unique only to Dilga). It should also be noted that, while Job is typically a straight upgrade with no downside (Lucian is the only exception I can think of), the currently available job enhancements often, but not always, involve a trade-off. So for the Holy Cavaliers other than Pieta, you're swapping out general PDEF/MDEF and healing for resistance to a particular damage type, which isn't always going to be an advantage depending on the map. Dark Cavaliers can't be summed up in a blanket statement since each job enhancement is so different, but a lot of them have either increased skill HP cost consumption, increased jewel cost, or a change in range/area. Don't get me wrong - these trade-offs are worth it, but it's just something to keep in mind as you read through these changes.

If an ability is changed, I'm only going to list changed stats. So for example, Weapon Crush -> Severed Blade. They're identical in damage, cost, area, and duration, so I'm not going to list that. I'm only going to list what is changed; but if I specifically spell something out in the original, and not in the new skill, then it means that the new skill doesn't have that (ex: Guard Stance gives PDEF, MDEF, healing, and evasion, whereas Diamond Formation trades all of that for Slash Resistance).

For stat boosts, I'm only going to list the real major ones, like MOVE 1 for Espion, and the built-in resistances for a lot of the Holy Cavaliers. I will say though that the job equipment is often statistically quite a bit better compared to the normal job, but this differs for each job enhancement. Holy Cavaliers tend to get a big HP boost. Dark Cavaliers tend to get a moderate HP boost and PATK boost. The job mastery bonus is buffed for job enhancements, but not significantly so. If you're not running the job enhancement as the main job, it is not, in my opinion, worth job enhancing and job mastering just for the ~30 HP you'd be gaining from the job mastery bonus for doing so. If you're interested in seeing the stat differences, you can play around with the unit planner here

I'm also going to give a brief opinion after I list the changes. I bounced a lot of these ideas off of /u/kahare since I'm not much of an expert on the game, nor its balance. So big shout out to Kahare for the help!

Holy Cavalier

For the most part, the Holy Cavalier jobs focus on providing resistances to a particular damage type. At level 20, they trade in 5% general PDEF in exchange for 25% resistance of a damage type. Holy Cavalier [Pieta] is an exception, focusing instead on healing.

Isaiyah

Focused on slash resistance/slash damage debuffing. Slash Res 21%

Characters: Vettel, Fraise, Monzein (when I have a character's name italicized, it means that the character will eventually get this job enhancement, but does not have it yet in global, or that the character is not available yet)

A little preface since this is the first one of these I'm describing... These damage resistant Holy Cavaliers have a built-in 21% resistance AND the option to use a passive for 25% resistance, for a total of 46% resistance of that type.

The strength of this is going to be map dependent, although slash is probably one of the more common damage types, so I'd say Isaiyah is better than average as far as the Holy Cavaliers go.

Porta

Focused on pierce resistance/pierce damage debuffing. Piece Res 21%

Characters: Teona, Lambert

The strength of this is going to be map dependent, although pierce doesn't seem as common as other damage types, so I'd say this one is a little worse than average.

David

Focused on strike resistance/strike damage debuffing. Strike Res 21%

Characters: Zeke, Reiner, Bado

The strength of this is going to be map dependent. A decent amount of monsters do strike damage, so this one is about average. This would also let you survive a hit from Supreme Battle Trance, albeit that situation won't arise often.

Magi

Focused on magic resistance/magic damage debuffing. Magic Res 21%

Characters: Victor, Gladiolus, Teresa

This one is probably even more map dependent than the others. A lot of maps have no magic damage at all, and others have nothing but magic damage. On average this is one of the better Holy Cavalier job enhancements, because Holy Cavaliers already have strong physical defense. Therefore, Magi helps round the class out by providing magic resistance.

Pieta

Focused on healing/removing status effects.

Characters: Chloe, Miuna Izayoi

Pieta is different than the others since it's more focused on healing rather than typed damage reduction. This is technically a straight upgrade - you're getting a new skill that doesn't replace an old skill, plus you're swapping out the very useless Clear Stone for a very useful status removal AoE. However, it's not nearly as good of an upgrade as the others for the situations where the others excel. The healing on this isn't really spectacular, so this probably won't make you go out of your way to run Chloe.

Ark

Focused on Area of Effect (AoE) damage reduction.

Characters: Alphonse, Edwin, Don Taras

So here's the funny thing about Ark... Ark reduces damage from AoEs, but to use it, you have to... be next to your allies... meaning... you get hit by AoEs. Not only that, I don't even think you can argue that both Pledge of Protection and Impregnable Formation aren't straight downgrades from their normal Holy Cavalier counterparts. It's so bad, that it might not even be worth upgrading a Holy Knight to Holy Cavalier (such as with Alphonse). /u/OriksGaming made a pretty good post on that here.

Ghia

Focused on agility.

Characters: Zain, Tyrfing

First of all - we don't have Ghia (also called Gear) yet, but it took me forever to add Ark, so I want to future-proof this.

Ghia is easily the best of the Holy Cavalier JEs. You're getting two brand new abilities that are both really good, and you're getting an upgrade to Light Bringer. Light Bringer becomes more expensive by 8 jewels, but it's well worth it for the AoE and damage increase. Meanwhile, Gear Break is a really nice ability in multiple ways: it gives you mobility through a teleport, gives you a huge AGI boost towards your next turn, and ignores defense (although the damage is pretty bad).

For your reactive, Gear Drive is more likely to proc than Holy Knight Stance 2 (which has a 40% chance at level 20), but I'd probably still run Holy Knight Stance 2 on most maps, especially where the enemies hit hard. Gear Drive is better on maps where you'd have a lot of enemies that hit for low damage, or if you're farming levels where you have no threat of dying. So the fact that it doesn't outright replace Holy Knight Stance 2 is a nice bonus.

Dark Cavalier

Unlike with Holy Cavalier, the Dark Cavalier job enhancements do not have any mechanical similarities. You're going to notice a trend with these... Dark Cavalier tends to not be a job you run as a main, so the best Dark Cavaliers job enhancements are ones that have good passives or good basic sub-skills.

Guernica

Focused on damage through casting skills.

Characters: Glanz, Suzuka, Yuunagi

Guernica doesn't really get a lot; it simply swaps Juggernaut for Blade of World's End. This is a pretty heavy hitting attack, but it's also pretty expensive. Even with a 5* hat, you're not going to be able to cast it on your first turn.

Jaeger

Focused on damage.

Characters: Leoniaz, Mei Fang, Itsuki

Jaeger substitutes some survivability for increased damage. There's not a lot to say here - it's a nice damage boost, but it's nothing spectacular. The passive is good, but not as good as Faust. It's also really only good if you're running Dark Cavalier as your main, since otherwise the HP cost reduction means nothing... but if you're running it just for the PATK buff, you're better off using Overdrive.

Zeele

Focused on absorption attacks.

Characters: Zahar, Balt, Annika, Blitz, Gilford, Niggle

Zeele gets a boost to survivability by swapping out Evil Slash with Dark Domain, which is like Rider Yomi's Blood Temple (though not quite as good). It does ignore defense, though, so it's a great way to do an AoE centered around yourself to kill metal ratties.

Life Eater is a nice damage boost over Blood Drain since it uses PATK instead of MATK, and also since it ignores defense. So this should provide for pretty decent heals. Zeal Eater will also be a pretty great way of stealing jewels from the enemy.

Zeele is easily the best of the Dark Cavalier job enhancements. The dark element doesn't get a whole lot of healing or regeneration, so boosting Life Eater helps round out that weakness. What's great is that Life Eater and Zeal Eater are basic abilities, so they can be run with another job as the main. That doesn't matter much for Zahar, who is going to run Dark Cavalier as his main anyway, but when Balt gets this job enhancement, Twin Swordsman with Dark Cavalier [Zeele] as a basic ability is a very powerful combination.

Espion

Focused on movement and evasion. 1 Move.

Characters: Ravina, Anastasia, Chat Noir, Rusha, Cordelia, Merlinus

Espion isn't getting a whole lot in terms of active abilities, but it makes up for that with its passive gains. 4 move instead of 3 is really nice, and the agility and evasion boost from the passive is nice for some characters. Phantom Assault is a nice way to get an even larger movement boost, and since it's a basic sub-skill, it could be run with other main jobs.

Dark Cavaliers are on the slower side at 3 speed, so Espion is pretty nice in rounding out one of their cons. This is one of the better Dark Cavalier job enhancements, but it's not necessarily a good option for Ravina, who is likely better off with just straight Magia Gunslinger Overdrive.

Faust

Focused on skills that reduce HP.

Characters: Ramses, Zofia (available soon), Masamune, Zeke, Ankh

Faust has a higher HP cost than a base Dark Cavalier. It's one of the better Dark Cavalier job enhancements almost solely for its passive, which can of course be used without running Faust as a main. Ramses will use the passive to boost Dragon Cavalier job enhancement that he'll get later, and Zofia will use it to help boost her Necromancer.

That's kind of an indirect way of saying the class abilities themselves are meh. The problem is that without healing available, the increased HP costs are really going to hurt if you intend on using those skills.