The fantasy WotC seems to be going with for Sorcery Incarnate is that of being a wellspring of arcane power. The party is facing a dragon or some other dangerous foe, so the sorcerer casts this spell and then for the encounter the sorcerer can burn through sorcery points faster to make their magic more potent. Maybe the wizard can go longer and cast more spells, but the sorcerer can put out raw power at a prodigious rate.

The problem is, how they implemented it as a spell means that in play it won't live up to the fantasy.

There's a few issues.

1. It runs into the rule about not being able to cast multiple spells on one turn. Because it's cast as a bonus action, the only spell you can cast as an action that turn is a cantrip, and you have to wait until the next turn to cast leveled spells. So rather than opening the encounter by throwing out your most powerful spell at the dragon, you open with a slightly stronger cantrip.

2. It takes concentration, which is arguably a spellcaster's most valuable resource. By the time you're a 9th level caster, your usual combat routine is probably "cast a concentration spell on round 1, then use cantrips or---if it's a difficult fight---non-concentration leveled spells on later turns". You could use Sorcery Incarnate to get the double metamagic benefit on your concentration spell, but then Sorcery Incarnate ends and you no longer get its benefits. That feels bad.

3. When you get it, it costs your most valuable spell slot. Why would you spend your only 5th level spell slot to buff lower level spells, when you could use it to simply cast a more powerful spell? (You can use sorcery points to regain your 5th level slot, but that eats up your bonus action on the next turn and uses up most your sorcery points, cutting into double metamagic, which is the main thing Sorcery Incarnate gives you.) Even at higher levels, a 5th level spell slot remains a limited and precious resource, and it won't be worthwhile to spend it on casting double metamagic a turn late.

4. This isn't about it being a spell, but let me mention it anyway since it's a big part of why it's not worth spending a spell slot on: very few high level spells use attack rolls, so the advantage on attack rolls does very little. The main benefit is you regain 1 to 4 sorcery points and can do double metamagic, which is not much for the cost of a 5th level spell slot.

One the other hand, making it a class feature (say usable once per long rest) would immediately solve issues 1 and 3. And it'd be easy to lift the concentration requirement and solve issue 2. In my opinion, it still wouldn't be a super powerful feature---how often do you find yourself wanting two metamagics on one spell? But it'd turn it from a feature that's never worth using into one that'd you use every day. Also, that would make it feel less bad to have subclass features, like the draconic sorcerer's dragon wings, tied to Sorcery Incarnate.