Early Pokemon games are sort of dated mechanically, but they do have a lot of freedom and simple gameplay which makes them easy to pickup and revisit.
Gen 5-6 have pretty polished game mechanics and quality of life, but stuff like the Game Corner, Battle Frontier, etc. starts to get phased out because they don't think it's worth developing anymore.
Sun/Moon is just hard to enjoy because the game never leaves you alone with the tutorials. They were really scared of losing kids to smartphones, Yokai Watch, and the like.

Never played the Switch games, but I will try Zygarde's game since they are changing up the gameplay.
I don't know how they get away with the Kalos girls looking like this.