More can always be said, especially about CK3's choices, but feudalism is a tricky subject and u/idjet discusses it at length here: https://www.reddit.com/r/AskHistorians/comments/1xwqqf/comment/cfflog6/
Their first post goes into whether the middle school textbook definition of feudalism is actually even a meaningful way to think about the complex relationships of a vast geographic area spanning a thousand years, and summarizes some of the arguments. The second post takes a stab at describing how things worked in practice with some examples.
The Byzantine Empire is a good example of where the game's mechanics sharply deviate from reality. I believe CK3 treats it more or less the same mechanically, but they were a continuation of the Roman Empire and as such didn't have anything like a "feudal" system for most of their history.
u/Guckfuchs discusses this here:
For much of its history the empire was divided into provinces with governors collecting a salary, with taxes paying for a standing army. This actually would play more like EU4! (I think, I haven't played it) It certainly lacks the medieval "feel" that CK is going for.
I encourage you to check out the CK3 Megathread with answers by a whole who's-who of flairs (I don't want to leave anyone out so I will simply mention the OP, u/hellcatfighter) on all kinds of CK topics, if you haven't seen it already!