While I agree that Sylvanas' undead persona had more freedom than Arthas did, it's still an undead persona that almost all people afflicted with undeath suffer.
take the Knights of the Ebon Blade. They had personality and willingly served the Lich King until he admitted he sent them to die. That's all it took for them to turn on him was self preservation. They're still dark dudes who kill and raise undead whenever they want, purging a bunch of dragons and raising them aswell in Legion. Sylvanas was raised and immediately tormented thus she was always against Arthas and the Scourge, but they held a tight leash on her actions unlike the deathknights. Darian and the Death knights that served the former Lich King who turned on him when he admitted he sent them to die are still being controlled by dark asshole versions of themselves and have yet to be freed from it and still kinda evil.
The evil undead persona isn't the real person. Regardless if they're free of the Lich King or independent, they are still being affected by dark Necrotic magic that has twisted them. Very very few retain their personalities, either because they were already evil or something like a strong faith in the light, or raised without the taint of necromancy like Calia. Not that I want to defend Sylvanas in any way, but this is something ive been arguing on here for many years and Blizzard seems to agree(except for arthas which they still condemn cause it would ruin him or something)