I consider them (Genesis mainly; I have less problems with Angeal) to be unnecessary additions to Sephirot's background that, in turn, diminish the original story and make it too convulted. Too many characters with strenghts unparalleled that emerge out of nowhere and have no consequence to the world in reality.
My beef is mainly with Genesis: cringey name and surname, pandering design (literally a J-Pop star, which is alright, mind you, but it feels like Nomura wanting to have an artist he's a fan of have a self-insert. The design is alright beyond that) and incredibly embarrasing scenes. I know it's the normal thing to hate by now, but the LOVELESS scenes, like the character himself, are so corny and pretentious.
Zack's story with Cloud and Sephirot should have been enough to fill CC, so these new characters also detract from that. It makes something that I consider should be simple enough to be needlesly convulted (which is not to say that the story is hard to follow, by any means -although I last played CC when it came out, so I don't really remember a lot of it-).
I get that this all sound like me crying "not muh FF VII", but seeing these guys you had no idea even existed (because they obviously didn't) play such big parts in such big moments in these characters' lives feel tacky and fan-fictiony in execution, almost. I guess it's the problem with prequels based on very specific characters and events.
Sorry if I don't seem very articulate or elaborate good points, but I'm tired as fuck tonight and it's been ages since I last played the game, so I'm just going by what I remember initially disliking of the character and game in general (if I played it today, I might even like some parts I originally dismissed; or maybe hate even more

).