He's not right at all, it's the kind of response you get when your class isn't the best overall spec in the game so therefor it must obviously be garbage. While I was playing I played with arguably one of the best shadow priests in the world, I'm sure he'd have plenty to moan about to but his performance was still crushing and dominant, the logs for the class over legion also show clearly that it was very strong without having to highlight the hilarious instances (like in EN).
My SP buddy is rerolling lock in BFA, but that's another matter. In Legion Shadows had nothing to complain about when you look at the big picture, but if you're in your own little closed loop circuit of shadow priest of course you're sensitive to issues, but you're deaf to how those issues compare relatively when looking at all classes as a whole and considering their issues too.
So like I said, he's not right at all. For sure you have something to whine about for BFA, but Shadow was amazing for Legion.