I think that depends on the game. I will say that I imagine it's important as sound director to make sure the person you're commissioning respects rhythm games and understands what "works" for them, and this probably feeds into the homogeny. but chunithm pulls in quite a few different artists all things considered. IIDX has its mainstays but generally the impression I get from observing the history of BEMANI games is that the sound direction aims to have a selection of trusted regulars and then a small rotating group of other commissions (who then become regulars if their work really takes off with the playerbase). maybe the ratio of regulars to new faces could be different, but personally I think this is the best way to do it - if somebody makes good music then it makes sense to keep commissioning them, after all. casthour aside, IIDX's new faces actually generally aren't rhythm game artists, like, who had heard of soejima takuma or sayohimebou in a rhythm game before...? many artists who are now popular in rhythm games got there in part from being regulars in IIDX, too, so there's a bit of an impression that IIDX is the trendsetter and other games follow, or at least IIDX was the trendsetter during the first 6 or so HD styles.

on the other end of the spectrum though you have Chrono Circle launching with just the most generic rhythm game artist list possible to the point I groaned when I saw it. like, it does make sense to commission ppl like skz and cosmograph because they're cool and popular, but if you only commission from the usual pool of well-known artists and no one else then the game's music will lack personality, especially if you're never having those artists step outside their comfort zone. Arcaea is another game where I have the impression it specifically panders to fans of what's already at peak popularity in doujin music right now, but it doesn't come off as cynical as Chrono Circle because Arcaea's commission choices craft a convincing musical aesthetic for the game, which is valuable. I will give also WACCA's artist list a pass because it's the TANO*C game and it makes sense all the guest artists are people who you'd see frequently appearing with TANO*C.

for what it's worth, I think it's good Iimori got commissioned in maimai. his style is oriented around bass music that isn't common in rhythm games and spreading him to other games isn't the same as commissioning a new artcore guy or whatever. related to that, I have the impression that Japanese rhythm games are hesitant to hire overseas artists outside of those like Frums and RiraN who became known through the doujin scene. I doubt there's lack of interest on the artist side - as far as western artists go, DDR is ingrained into western rave culture and lots of acts are probably outright influenced by rhythm game music, I am sure VOLANT for example would say yes if commissioned for a popular rhythm game. recently, kors k's influence on DDR and DRS is cool because he actually follows the EDM scene and I imagine that's been the reason those games have more currently popular acts and overseas artists. Stessie in DDR is a huge boon. kors k also follows camoufly on twitter, will camoufly come to Dance Rush???? (I hope so)

ultimately all I really ask is that sound directors keep up with music outside of rhythm games and scout for people who would be good fits. even within the most mainstream doujin scene there are regulars who don't show up in music games often but would be good (from diverse system, 100-200+tanigon are cool, and I would like to see MAYA AKAI appear in games more). I'm also a proponent of more genre variety. I think IDM and juke make sense for rhythm game charts, and just more instrumental music would be good. fusion is underexplored outside of GFDM. there's no reason why you shouldn't be able to play a rhythm game and hit buttons to a mario kart sax solo instead of supersaws.

as an aside my ideal rhythm game sound direction is new songs from tarolabo, Bitplane and yujurie in every game (actually impossible)