The detrimental effects that stack are ones that just deal damage. If you couldn't attack someone with your melee combo as a DRG the same time another DRG is attacking that target, that'd be stupid as hell.

It's the same for healers. You can apply multiple regens of the same type to the target, you just can't benefit from the effect more than once, because suddenly 3 ASTs could give their team battle high 9 every 90 seconds and it doesn't take much to one shot groups of players with instant aoes at that point.

Likewise with SCH DoT, it does damage and it'd be real dumb if only one SCH got to deal any damage other than Broil, but the Biolysis effect, the one that reduces damage dealt by that target, does not stack, because that'd be far too powerful.