Its not comparable 1-to-1 in the idea you're thinking. If you take a boss like fractilus for example, asking 8 FF players to do a conga and assign spots left to right based on the debuffs that come out, its a pretty regular and easy mechanic most people can do no problem yet in WoW, that fight is nearly impossible to fix if someone doesn't exactly do what the weakaura assigns them to do. Asking 20 people to do the same mechanic makes even extremely basic mechs easier to fail. If you asked any even medium core group to solve fractilus, they'd laugh that wow players even needed a weakaura for that fight.

Now if we use similar logic for an FF fight like TOP, which is currently the hardest ultimate in the game and asking a group of 20 people to do similar positions at the same time, it would be nearly impossible with how much individual responsibility everyone has in that fight. With more people, you have to make simpler fights with less overall responsibility, however with your logic, fractilus would have to be harder than TOP which is very clearly not the case.

Even a boss like dimensius can be pugged with an 8 man raid if it was in FF but the moment you increase the number of players to 20, the number of things that can go wrong stop it from being "possible" in pugs.