In regards to the precise nature of the the empires feudal structure... it's complicated.
@tricholysis described the otl situation pretty well, but ultimately the Celestial Empire isn't really an exact mirror despite cultural trappings and some other similarities.
Ultimately, this boils down to the fact that even running on a high tier protagonist grade bullshit, the Sage Emperor was only really able to weld a half dozen odd neighboring kingdoms together under fairly grudging circumstances, and while many of the original ruling families are no more... well that just makes the ones that are still around even more jealous of their power and privilege, and the newcomers, for all that they might be willing to make concessions in the name of their own power... well they
want that kind of power and privilege, there are limits to how much they want to let that erode.
Like... take the Bai for example, they are getting fucked about a lot in their moment of weakness. Meizhen's mother, the scattering of their scions, etc. But the fact remains that the Imperial Court has to measure their moves carefully, because if pushed to far Meizhen's grandad is absolutely capable of doing the angry asian patriarch version of raising both middle fingers and saying 'come and have a go if you think you're hard enough'
...And the court can't be sure that the Zheng and the Xuan wont give them the side eye if they actually went all in on extermination, even ignoring the whole Ancestor problem. After all, if the imperial court is so brazen and domineering, they might be next, maybe the folks talking about the decline of third dynasty under Emperor An's father were onto something.
The same basically filters down into ducal politics. The nature of a Xianxia setting makes truly centralized government hard, especially since I've gone out of my way to ensure that you need a pretty hefty cultivation gap to be
completely invincible to large numbers of lessers, if you don't have your own backup. Cai Shenua broke a whole lot of rules and traditions, and she's in the process of breaking many more... but if she didn't ultimately have backing from at least a significant minority of the count tier clans, she probably wouldn't have succeeded in her little coup. Though of course, not everyone got what they were expecting out of that backing.
So yeah, there's a lot of careful dancing around between lord and vassal, because the lord doesn't generally want to be seen as shitting on his vassals too much, or without a good reason, because that riles up all the other ones, and that can be bad. It all depends on the power disparities involved, and which province your in, and a dozen other factors, which makes any effort at actually codifying the rules... difficult. The Gu are a good example of this. They're powerful, wealthy and well connected enough that the Han Patriarch doesn't feel comfortable just stamping on their ambitions arbitrarily, and in turn the Han are powerful enough that the Guo are reluctant to do any blatant stomping either.
...Well that went on a bit