It's in their nerf scope, but the debuffs are what boosts the potency. It's why people are crying for MCH to get nerfed because they don't know how to handle a good MCH, much like they wanted SMN nerfed and want AST further nerfed.
Most I've done was 13k using full burst, so it is definitely powerful lol. So not every MCH will pull off even more than 3k damage with WHILE using buffs, BRDs are even less likely to deal that much damage, but it's mostly the good MCH you have to watch out for.