I know with a title like this you're probably rolling your eyes but I'll try keep it constructive.
Now I've been playing since late Vanilla and I've transitioned to a different spec and class almost every single expansion and I've been around and heard and seen most of every major change. From SM/SM Warlocks to dual weild unholy DK tanks to StM in emerald nightmare, I've seen a lot of shit in this game.
And in my years and years playing this game I have never seen a worse spec in this game than Shadow right now. It's not that the abilities are bad or are badly designed or that the class is clunky, it actually all works together smoothly and gels pretty well. The problem I find with Shadow is that it is literally nothing but a one trick pony. This spec does absolutely nothing "alright" to "great" except DPS in a prolonged raiding environment.
So in a raiding environment, you start off hitting like a wet noodle until you finally start gaining void form and lingering stacks after everyone's all popped their burst. THEN you can begin to start doing average damage. That's kind of where all of this class' problems root from. The idea that it has to ramp up slowly before it begins to really pile on the damage. Which in theory is perfectly fine, it's a unique playstyle and something very different from the rest of the classes. The problem however is that in practice and reality, this bleeds out across the rest of the game creating a balancing nightmare for blizzard. If they give us burst or faster damage, that makes shadow's ramp up overpowered. When you have a balancing nightmare such as this, it makes it impossible to be good at anything in the game except what your spec is pigeon holed into, a medium to long fight raiding spec.
This class has almost no AoE, no burst whatsoever unless you setup for it 2 weeks in advance. Its strength, which I assume Blizzard would have us believe is prolonged cleave fights, isn't surpassed by other specs. Its utility is reduced to a heal which is seen as a nice bump and not a serious major raid cooldown. Dispersion is cute for soaking but many other specs are capable of soaking just as well.
Because of the reasons above, shadows mythic capabilities are severely crippled because of the severely high ramp up time. While the spec is much stronger at higher keys, it's no better than other top tier specs. It doesn't bring anything different or unique to the dungeon that anyone would ever think to themselves yeah I'd really like a shadow priest here. Forget lower mythics or doing your daily heroic as shadow, you'll get about 3 ticks of swp before a mob dies.
Solo content is good but there is a severe lack of quality of life because of the ramp up problem. Our main iconic spell, void form, is useless for questing and moving around locations, there is no movement ability, no mini tank pet, no get out of jail free card, nothing super unique to shadow that other classes can't just do better. It's crazy how much survivability warlocks have in PvP as a caster with 0 movement yet priests are expected to be able to function there too while having the movement of a warlock but the survivability of a mage. Some kind of ethereal blink or void cross over that makes you go invis/stealth for a short while (which we had) would be great.
I legitimately think that the best thing that could ever happen to this spec would be to make it its own unique class or create a 2nd shadow spec. A direct damaging spec and a dot spec. Because the way the talents are designed now is that it forces you into this ramp up style of play which I think is well designed in theory but in practice results in poor quality of life, average damage at its best and completely useless everywhere else. I know they're trying to curb the ramp up in BfA but the focus on any ramp up at all results in this happening across the game.
Anyway, it's late, sorry if it seems a bit jumbled and messy but I adore this spec so much, I think it is leaps and bounds ahead of any other spec in the game for its lore, its richness, its complexity, its aesthetic. And it's just SO let down by very basic things like no movement and being a one trick pony which if you take anything away from this, it's that.
By being so focused and built around ramp up damage, it creates a balancing nightmare wherein the class becomes garbage in every other aspect of the game that isn't a medium-long raiding encounter.