I haven't heard of actual invincibility being a problem but people are apparently able to edit their MP regen rate to always be at its "out of combat" pace which gives them an unreasonably high amount of extra Recuperate usage.

Purify is subject to server ticks so there is a brief window after you remove a status effect, but before the CC immunity actually kicks in so if an enemy team is actively trying to chain CC some of it will inevitably get through.

There are definite balance issues, and as expected people have caught on to them and started gravitating to the objectively better jobs.