For me Genshin Impact and Hades (including Hades II) are two examples. These games have relatively simple art styles in comparison to e.g. the realistically painted art of certain Blizzard games (WoW, Hearthstone hero portraits), but they are hard to learn even to someone more advanced in art creation.
I haven’t played Genshin Impact, but from what I recall from trailers, there are many anime game with similar art style. I am currently playing Atelier Ryza, and I believe it has similar art style too.
Even with Hades, I don’t think the art style is difficult, but a lot of effort has gone into visuals of that game. There is lots of detail in everything. So, maybe in that sense it can said to be hard to reproduce.