Sephiroth's story in 7 is essentially "he was the best of the capital S Soldiers in the war". Then he finds out he's a byproduct of monster cells and flips out and burns down a town, and gets fluke murdered by a kid. In itself, not terribly different from Kefka, although Kefka becomes a willing participant in the whole Esper-experimenting thing (while other MAgitek knights were unaware (Celes) or ignored it (Leo)).
Kefka's motivations being an insane power hungry monster (who then becames a psychopathic insane god) are of themselves, nothing special, true enough. (He's also not the first villain to "win", so did X-death, except the merged world wasn't as bleak and we mostly ignore XDeath because his name is XDEATH)
Kefka interacts with the world around him and builds antagonistic relationships with cast though. Sephiroth does not. He has a backstory which is a flashback in which the parts where he interacts with Cloud didn't actually happen. He goes crazy and burns down a town, then gets plunged into the ooze. Its been awhile but IIRC he never even sees Clouds face under the soldier helmet, so he should probably actually remember Tifa better. Cloud sees Sephiroth in all the black hooded white haired clones because of his own PTSD and hallucinations. But his own personal enmity with Sephiroth is just as fake. Zack might've had a relationship with Sephiroth to merit a rivalry, but Cloud does not.
Sephiroth can't independently carry out Jenovas legacy... because Jenova is still around and driving all the events. All the actions of the clones from kiling Shinra, to taking the black materia, to killing Aerith are Jenova's direct handiwork (often including a direct battle with Jenova immediately after). Because of their insistence on writing a convoluted twist in, Sephiroth is present for about 15 mintues for a final boss battle, endcapping a rivalry that makes no sense because he shouldn't even know who Cloud is, and Cloud getting past his PTSD and delusions was already resolved before the final chapter.
Kefka's motivations might be generally weak, Golbez and Kuya spend most of their games as Vader to the unrevealed Palpatine characters. But they weren't sitting on the toilet in the final dungeon the entire game while other people pretended to be them and characters hallucinated their existence. Ultimecia is probably the closest matching villain, but FF8 never even makes a pretense that she's personally relevant to the characters (and she's also at least the driver, not the slave)