Disc Priests don't have a healing cd, only mitigation, if disc priest mis-times that 'oh so precious' bubble dome, it is worthless. As is if they spam sheild everyone and it doesn't actually absorb any damage.
I'm sure priests still top meters even if they are just healing tanks. The reason they top heals becuase 'overhealing' does not factor for them.
Disc priests will always be strong on fights that require low healing. Which is probably where this complaint is coming from, easier, lower-damage content. In more difficult content, as damage can spike, is where you can determine for yourself that Disc priest is not One-bit OP.