That depends what "hard" is. I've seen soloers say raiding is easy because you have 10-25 other people to count on. I've seen raiders bemoan the ezmode nature most games acquire over time.

IMO, the combat and enemy AI in most MMOs is too simple to ever be hard, at least in the same way that other video games are hard. Combat (excluding critical hits) is a foregone conclusion if you pay attention. Your character can beat X enemies of Y level at once. Even grouping/raiding boils down to choreography rather than thought most of the time.