Every armor set should be at the level that the Invader set is.

Some ideas:

Sentry

• should just get a flat, constant damage buff against Taken
• should either be immune to Invader wallhacks, or the marking ability should be permanent even after death (and possibly just by aiming at the Invader like with Foetracer)
• should enable the Well of Light to heal everyone like Healing Rift

Reaper

• multikills should also begin health regeneration, or restore a chunk of health like what Wormhusk Crown does
• killing a powerful enemy should grant quick charging of all your melee/grenade/class ability energy, and grant some bonus Super energy

Collector

• you should drop all your Motes when you die, or perhaps you drop half and respawn with the other half already collected and ready to bank
• instead of 5 Motes = overshield, I think 3 Motes = restore a chunk of health
• possibly, the "Cashback" skill could also allow you to occasionally pick up 2 Motes from 1 Mote on the ground. Like a 10-20% chance

I think stuff like this makes the other roles better, in such a way that you'd miss them if they were gone. Sentry would be more powerful against the Taken and as a counter-invader, Collector would be a more risk-free hoarder of Motes, Reapers would be more efficient at just slaying the add waves.

You should be able to think "man we kept getting wrecked by the Invader because we didn't have a Sentry" or "man we would have had more Motes but we didn't have a Collector" or "we could have killed things quicker if we had a Reaper" instead of it mostly being about how successful the Invaders are.

It's entirely possible to come up with other ideas by really reworking the set bonuses but with this one, I tried to keep it mostly the same flavor just better.