Yeah, I mean the dps spec of a healer (switching to healer on dungeons), usually the ranged (maybe the cat form has been good on a few expansions, but its a rogue, really, and I have one of those). Both shadow priest and boomkin were hard plays in recent expansions, maybe the gear drifted, I couldn’t get past the first major set of pulls on a shadow priest in Shadowlands with still having mostly Bfa gear on. I came back with heirloom gear on and it was fine, but I did need to flash heal quite a bit on a few serious pulls. I last remember Boomkin being a good spec perhaps as far back as Wotlk but its been a long time since its being an easy spec to do.
Elemental Shammy has been good for a long time, it seems to be the only dps of a healer who seems to not “just die” when you pull 2 mobs (something which a mage pretty much always does). It has a good burst and kill, rest have been disappointing in that way.
It’s pretty much been the same for classes as to which one is easy or hard to level since draenor, the dps classes are mostly hard, and the tank can dps as well as them (far better en mass) when levelling.