They linked DPS and Healing, which means that if you want to do 100% the same output as other healers, you'd need a mix of DPS and healing to be balanced. But if you're not doing 100% as much healing as other specs, you're worthless. And if they buff the healing then you do over 100% output, making you OP. Same with DPS.
So they made the DPS the weakest out of all the Healers to justify having good Healing output. But then you have the problem that most of your rotation (all the DPS spells you cast after applying atonement) feels like you're slapping a wall with a wet noodle. And you get outperformed by a Holy priest that just bursts everyone full and then does a DPS rotation. This is why the top healers are the ones that DON'T transfer DPS into healing directly, but instead stop healing entirely just to DPS.
It wasn't always a problem, but M+ made it worse.