Yea I think there’s some end-game experience lacking since classes are not at all about using your skills, only, if you are in a group. There’s extreme gaps between good tanks and bad in the game, as well between healers which would be odd if it was all so trivial. It’s similar as if someone said oh soccer is not complex, seems you only need to kick the ball, right
You can have only 1 single combo (provoke) and be overly occupied with looking around and deciding constantly what to do. It takes alot of practice to be aware where to tank groups, how to separate them to get maximum efficiency etc. (ex. with no shamans in the group the melees could be pulled away for rangers to hit the ranged mob first) and all does not help if not every dmg dealer in the group also learned what to hit when.
While mages can practice alone for healers and tanks it is all about positioning and timing.
Good groups don’t only know their rotations and how to use their skills, they are good in movement and coordination, awareness of everyone’s role etc. It is a huge difference if the healer is aware whom to heal when and how much or if he just dumps his stuff on the ground. As for talents and rotations, Musky is one of the most complex classes since endgame healers need to constantly decide a) when to overcharge the turret, when not and where to place it, b) when to load the turret (with orbs) and what to load, c) when to charge-shoot either turret or mobs (a ton of muskys overlook that gamechanging talent completely…), when not, etc. etc. They need to know how paladin and warrios work and what their possibilities are, best play them themselves, to adapt to them; they even need to adapt to personal styles of tanking which can take a while. Some tanks play differently than others, their skill level and rotation also decides how to heal, for example to not pull aggro. As well as for the Bard. Less choices than musky, sure, but there it is all about timing and partly, positioning.
Very hard content (ex. temple 15) we managed with healer and paladin constantly telling each others when they used their heals to coordinate, since other than warrior there’s no clear indications.
As for shamans there’s several things which need alot of practice, popping your runetiles first of all. Also it is a huge difference if shamans know how to move or if they position themselves so they regularly block rangers or take bolts. Good shamans can tank ranged mobs and hit 100% of their orbs from a distance which is not too trivial to manage on smaller mobs. They can throw totems too early (like lava which got a long respawn time), before things are positioned or hold them back, also if a group is almost down, which can make a huge difference for the next group.
And whatever they do, it only works with a tank focusing on totems also, since he easily can pull stuff away from them; the best tanks in the game are always thinking about rangers AND shamans alike when they position things.
It does not hurt to have some more classes, sure, yet underestimating the complexity of the existing ones, specially if you did not play all yourself in harder endgame content yet (lower is indeed trivial for good groups because mobs are down too fast to even need strategies involving tanks or healers) is a common mistake since most of these things are getting relevant only later.