I think high end raiders can find shadow priest fun because its feedback systems is one of the best in game. Assuming you don’t run random proc traits like Infinite Stars, Shadow’s damage is very consistent pull to pull. It becomes much easier to judge your performance, and the simplicity of the spec makes it easier to point out areas that need improvement. Hell, your void form length directly correlates to maintaining your ABCs of a caster. Its a solid spec for min-max style of approaching the game.
But this is only in raid. While spriest can perform in M+, it often struggles because it relies heavily on momentum. It directly conflicts with other playstyles in M+ and its unreasonable to expect your tank to play around you and not the 2 other DPS in party. However, the higher in keys you go (20+), mobs just live longer, and momentum is less of an issue as there tends to be less downtime overall. You can watch someone like Matthxw2 on twitch pull his own weight in higher keys, but however hes not really pushing the 24s and 25s like some people are now (and each key level is a huge difference). Shadow is also propped up in M+ by Focusing Iris and Breath of the Dying (which is just a better and functional SWD).
As far as my issues with Shadow, its always felt like the situation had to be adapted to me, not vice versa. Raid members would have to play around what I needed to perform, as losing a GCD over the last 20 seconds of a void form can be as impactful as open GCDs during a combust for a mage. I originally started a spriest in TBC because I liked the utility of the spec. It did crap damage, but its shadow debuffs and mana generation alone justified it for a raid slot. Now its on the complete opposite end, feeling like it adds nothing but damage which your team has to play around. And it can’t even apply extra pressure when you need it, especially when so many fights are heavily reliant on it.
Which is honestly one of my biggest problems with the spec. Its always been a negative side to shadow. Hell, In HFC, shadow got boned pretty hard by the legendary ring effect. But its spread DoT cleave justified bringing the class. But Blizzard doesn’t like that niche being so strong, so where does that leave shadow? They carry this heavy burden of ramp DPS that offers no particular strength. Sure its consistent damage, but consistent damage doesn’t really have much value in WoW.