So, I'd like to touch on this a bit as someone who's been an
active WoW player for the better part of the past two decades.
The endgame content in FFXIV is plenty challenging, even for WoW players. I know this firsthand, having brought people from our WoW guild to our FFXIV FC to try their hand at raiding in this game. Some of it can feel even
more challenging at times, because it often requires different skillsets than people in the other game are used to. FFXIV raid fights are like a orchestra you sign up to be a part of and, if you fail to be playing your exact part at the time and place you're supposed to be playing it, you run the risk of ruining the entire composition for everyone else in a way that often isn't as easy to salvage.
WoW raid fights, on the other hand, are more like jazz. You're given a class toolset and a list of boss mechanics and your focus is of course sticking as closely to the gameplan as possible, but also improvising often when the need arises. Sometimes a person dies and a res isn't handy. Sometimes a boss mechanic clips a channeled cooldown of yours and you have to use another one earlier than you'd planned. Sometimes something isn't interrupted or CC'd when it needs to be and someone else has to step in and make the adjustment. In FFXIV, you learn how to do most fights the same way every time and learning the dance is where the challenge comes from. In WoW, two different weeks of fighting the same exact boss can feel wildly different, because there is a lot more variation in its fights and in what different class skillsets can bring to those fights.