Well,
personally i don't like that system anyway. But thats a different matter. I think it has been established for D&D to avoid stacking too many boni. I would have chosen a different approach though.
It feels indeed that NPCs have it easier to maintain it than the PCs. I think the war caster feat does help. But some instances are just broken.
I was casting haste on my char while inside a darkness spell. I ran out but had to move back, removing and reapplying the blinded status, which broke my concentration without even rolling. Not sure if thats intentional.
