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.
Being blind shouldn't break concentration and at least for me having darkness cast on me by some enemy didn't break concentration. So if it happens by moving in and out of darkness that sounds like a bug.
