You realize the sims are available to anyone who owns a computer right? An object that 99.9% of people playing FF14 will likely have in their household?
One of the statics I'm helping with TOP has several console players, and they all use the sim to practice the phase we're on. The sim even supports controllers, so you can just plug & play to truly emulate the experience.
If a mechanic's difficulty is solely based on whether or not you know anything about it / being forced to spend hours getting deep into the fight & wiping 50 times to sort out all the 'gotcha!' instant kills, it was never a hard mechanic in the first place, it was a tedious one. There's plenty of savage / ultimate mechanics that require problem solving & high execution even if you've trained on the sim all day, like in TOP alone we have Playstation markers of P2, basically all of P1, lots of P3, Sigma and Delta trio due to the plenty of RNG involved in the mechanics and relatively short time frames in which to execute the mechanics.
One of the best examples of difficult mechanics this game has had is actually in DSR, where despite being the very first two mechanics in the fight and as such can rapidly prog on both of them, Strength & Sanctity of the Ward have very high execution requirements that delayed pretty much every streaming world prog group for the better half of day 1 on its launch. No tedious 'get 9 minutes into the fight' bs to them, just strict execution checks that tons of groups wiped to even long past getting deep into the fight.
There's also plenty of factors the sim itself doesn't teach, such as simulating what actual damage is like during said mechanic since it uses random values so your healers still have to learn any nuances to healing during said mechanics, or simulating what sort of mit you'd need for certain mechanics. You could practice Gigaflare's edge movement all you like as a healer, but you'd never know what your resource charges/instants/mits look like until you experience it yourself and have to heal on the move.