It’s not so much as it’s difficult per say, as it’s just completely unclear in a lot of aspects, which you can argue makes it more default because of it.
Lets take a look at mechanics entirely, most of them are so unintuitive that discerning what isn’t and isn’t a stack is insanity. Most of WoW’s mechanics boil down to “there is a swirl on the ground”, well you can either stand in it for a buff, stack on it for raid wide damage, not stand in the “fire”, etc. There are so many options despite the same visual representation that it’s a mess, not only that there are some bosses within raids that do the same thing visually but a completely different mechanic, or things are tied to “what type of magic the boss is using”, some are red, or blue, or green.
WoW needs to adjust how it handles Raids entirely, and I know a lot of people are potentially going to give me hell for it, but FFXIV does a good job of making it clear in most cases what exactly needs to be moved out of, what a stack marker is, where you need to stand, etc.
Lets compare two bosses from each game, we’ll take a look at Raszageth and Hippokampos from P2S. And we’ll even look at something simple such as LFR Ras. Ras has a swirly that will slow your character down that needs to generally be placed in the back, a frontal beam (which has a very obvious indicator), and bad swirls all over the ground, along with being knocked back (a variety of this), a push back, etc.
Generally you want to stand in the bad stuff to avoid the knockback, but not stand in the bad stuff to avoid the lightning, but also accept the knockback at specific locations to land on platforms, but the one knockback is slow and you just have to stack on top of her or at least pop certain abilities to run around. While all of this is a very simplistic way of describing the fight, there are several knockbacks that all have different visuals, do the same thing, but require different positions and adjustments to it, while some of these are very obvious as to which is which because of phase transitions and which phase you are currently in, it’s not very intuitive for someone new trying to learn the fight.
Now if we look at P2S from FFXIV for instance, you’ll have tank busters with a clear marker, you’ll have arrows for which direction to slide to and where you need to line up with a partner of your own that has the opposite, you’ll have orange aoe markers to move out of, you’ll have specific debuffs to move away from the party, etc. It sounds like a lot and the only real comparison you can draw is “Don’t stand in the aoe markers for FFXIV and don’t stand in the swirls for WoW” and that is true, but it makes every other mechanic in most cases very generally visible, sure there is some trial and error involved there too, but it makes it very clear, then its up to the player over whether its a skill issue or not, and sometimes latency/netcode screws you over, but generally speaking its mostly fine.
WoW fights have nothing like that, if a tank buster happened in WoW, you have to pay attention to the abilities and pop accordingly, or just take it and get healed. For FFXIV, you get markers over your head over what is clearly a tank buster. Now in FFXIV’s case those sometimes look like stack markers, but again, it’s a very clear indicator. WoW largely has nothing and it makes the fights harder because of it.
Nobody is asking for them to be a constant 123 of abilities that never variate, which a lot of bosses are already like that anyway, but they need a lot of indicators to better smooth out the flow. Because there are bosses that are vastly more complicated than Ras by a large mile, look at N’Zoth and his mechanics for instance.
Also they should just tone down the difficulties in general. You don’t need LFR, Normal, Heroic, and Mythic. Just have it be Normal and Mythic and balance accordingly. Normal is basically the equivalent to FFXIV’s normal raids, and Mythic is equivalent to Savage or Ultimate raids, or what have you. You can have a lot of mechanics but they need indicators and better flow. That and they need to fix how gear is handed out, because as it stands right now there is zero point in raiding when you can get everything from M+, even with the new system coming, why should anyone bother with Mythic raiding when you can still do something vastly easier, vastly faster, and with less people. Also remove the trash mobs from Raids and have you spawn at the boss when you die, it’s a chore and raids take too long.
tl;dr: Hot take, probably gonna get a lot of crap for it, but it is what it is, read it and form your own opinion and discussion around it.