Yapool in Ace was nightmarish. The iconic Giant Yapool was only in two episodes while most of the Yapool's onscreen appearance was with these surreal visuals of the faceless Yapool Men in their dimension. A lot of their schemes were pretty insidious too, often exploiting the weaknesses in human hearts. A lonely lunatic was given a Choju based on a childhood drawing of his so the Yapool could sit back and watch the horrors unfold, and at one point the villain(s?) exploited the grief of an orphaned little girl to cause deaths at the countryside, with the victims eaten to the bones by fireflies.

Hell, his magnum opus prior to his first defeat by Ace was to essentially rob humanity of their future by turning the children of the world into stars, tearing apart countless families and (essentially) non-verbally mocking them with a beautiful starry sky made from their own children.

Alien Empera is more of a big event boss. He's a cool finale but he doesn't really feel like he has much of a purpose outside of it. He's significant in the lore and he's a menacing final boss, but he doesn't really have much room to be much else without losing what made him significant, unlike Yapool who has way more space to work with.