Levelling is cancer. If you go shadow its just harder to level as you dont have the health or damage to reliably heal yourself so consign yourself to pulling 1 or two mobs at a time.
If you go disc or holy youll never die but itll take forever.
For end game content for shadow we are now a hard CD based class. Void form is up once every 1.5 mins and this is our damage window and we are single target gods in this window, stacking every CD we have into it.
We have a resource, insanity, but its not really a thing any more. It was introduced as the primary control mechanism in Legion but these days its literally only spent to cast one spell so its more of a gimmick.
Because of this We are great in raids and really valuable on Tyrannical in Mythic+, but we really suck at AoE, even if we spec for it. It can make pugging Mythic+ which is more AoE reliant pretty annoying to outright impossible as Shadow.
For farming solo context at end game, itll be tedious for the same reasons as levelling until you hit a solid gear level, at which point it flips with almost no middle ground, to absolutely trivial. The sheer damage pumped during Void Form or healing through attornment combined with the fact we self heal through damage means once you hit a pivotal amount of damage youll be able to pull huge groups and solo pretty chunky elites and end every pull on full health.
For Holy I mean, its a top grade healing spec its exactly what youd expect.
Disc is a right ball ache to play, they seem intent on making it progressively harder and harder to play every expansion, which is impressive considering it was easily the most technically demanding healer after the legion changes by a fair margin. But that being said if you can master it its in high demand for all end game content. It performs exceptionally well in Mythic+ if you can master what is a very difficult AoE healing environment and its mechanics with Spirit Shell mean it can fill a valuable niche in end game raid content that is not provisioned by any other spec.
PvP wise, disc is no longer the psudo mage who can heal. Dps is pretty trivial but still top end competitive with other healing specs. Shadow is how its been for years now. To dangerous to let cast but trivially easy to lock down. In PvP as shadow youll have two binary experiences. Absolute tedium where you are correctly identified as a threat in which case youll do literally nothing for the entire BG or arena but die over and over again. And when they dont identify you as a threat, you blow every last CD you have and kill 6 people in 10 seconds and no thats not an exaggeration.