Ultimately the reason Disc Priest is C-Tier is their immense struggles at keeping groups topped up and having enough throughput healing on a per-pack basis. While tanks are getting stronger as a whole this season, they do still require throughput healing to keep them propped up. This means that Disc has to throw more Shadow Mend
s into their target to keep them alive.
Additionally, Disc does not have great upper bounds for potential damage output as a lot of their damage is passive. While this is great for raid as it generally translates to a lot of free DPS, for M+ maximizing the time you can spend DPSing from the healer position is an integral part of the game. That coupled with their lackluster HPS throughput are the reasons they are C-Tier.