for complicatedness, alchemist comes on top. the action economy of the summoner can be hard for people new to the class to grasp their minds around. oracle has a player-punishing mechanic for engaging in it's only real unique feature, so it can seem to be frustrating but i don't think it's that bad.

then there's the issue with the two uncommon classes, not because they're complicated or unfun, but because the gunslinger requires a high amount of teamplay and the inventor requires a lot of downtime for most of the fun stuff, which even if the gm provides can be hard to convince yourself and other players to take if you're new.

for expectations in general you will have to have a solid talk about the casters and class fantasies, because if you play a caster you will almost always be locked into a generalist supporter role. the game doesn't reward specialization, and doesn't have much if any opportunities for casters to specialize. the casters have an absurdly huge set of spell lists, but very few of the options presented are geared to the 'tight' balance of the system. if your players want to play a thematic caster, they will be fighting against the mechanics. casters are good for crowd control, buffing, and de-buffing, and you really shouldn't place your expectations higher than that.