Xemnas being a good boss battle and not much else pretty much describes all of the KHII Org. members (bar Xigbar) in a nutshell.
My big problem with Xemnas over the other members (who obviously could have been used much better) is that he is supposed to be a big deal.
Not only is he the main villain but he actually has been built up ever since 1FM. In CoM he is shown to be someone the other members fear (Vexen and he even brags about his own high rank)
Then you get to KH2 where Xemnas... doesn't do anything. Like, literally. See him very briefly two times in early game where he doesn't do anything, appears once in mid game where he is like I know about Riku and bails (which already was done by Saix earlier). In final confrontation his only kinda interesting part in the game is short dialogue with DiZ and then the game kinda ends.
In comparison Xaldin and Saix are much better as actual villains in the way they interact with characters (Beast, Kairi+Sora) and Xigbar is interesting in his lore foreshadowing and is a likeable goofball. Demyx and Luxord are the only ones I would put on par with Xemnas that do very little. Axel is watered down from CoM but at least he has agency.
In comparison Master Xehanort in BBS does a LOT. He himself does his dirty work and has actual plans you see unfold. Ansem appears very little in the end but he too has good foreshadowing for his later appearance and actually becomes major player for the plot before just going ''yeah it's time for final boss battle I guess''.