I mean, IMO algebra problems can require very clever insights as well, and number theory especially has some really nice proof arguments you can make. It's easier to make a bad problem of this category though because it's much easier to hide the difficulty in a bunch of computations / rote deduction, and not creative insights.

Combinatorics problems are usually simple enough that anyone can understand and try tackling it though, and the solutions in IMO are usually designed to be elegant. I don't think I've ever seen a bad combo problem before.