The only issue I have with Concentration is how easy it is to fail the Con Save, due to the minimum DC: 10. I would much rather total all the damage taken in a round, and have the caster make a single save at the start of their turn. It's more paperwork, but right now swarms of mooks doing 1 damage each is super strong at disrupting spells, which seems odd to me. Having a concentration spell fail in such a manner (especially after 1 round or less), really sucks. Given that only Sorcerer and Fighter (Eldritch Knight) are proficient, IME it happens far too often.