Crusader Kings 3 really has a lot of problems, some of which are very fundamental. I never played CK2, so there wont be any comparison to that game, its more a very objective review.

One of the biggest flaws the game has is the lack of difficulty, which partly is due to PDX trying to appeal to a broader audience (which is good ofc) and because of that only having very simple and few mechanics in the game. Sure this is fine and makes the game more accessible to a wide audience, but it really gives you barely anything to do.

Another reason for the lack of difficulty is the lack of threats the game provides. The game almost never throws something challenging at you, neither in form of a revolt or something like a plaque. It also is very easy to manage your succession in various ways, even tho some would say otherwise for whatever reason.

The last and biggest reason for the low difficulty is in my opinion the horrible AI. The AI is terrible at doing almost anything, the way they prioritize how to move their armies in a war is just flawed e.g. they split up if they are overstacking which is ok since they otherwise die to attrition, but they also suicide their armies all the time because the amount of soldiers in the opposing army has a higher weighting in the pathfinding than their quality. They also almost never declare war on you or try to kill you or your dynasty, even if you are super weak and do nothing.
The AI also fails at managing their realm, resulting in collapsing all the time. It got better since release by a lot, but its still super rare to see anything thats bigger than a duke and even if they become a big realm, they collapse because the AI cant handle factions or their economy well. Whenever they hire Men-at-Arms (MaA) and are in a deficit for just a couple of months (e.g. because the MaA are still reinforcing resulting in higher cost till they are done reinforcing) they just disband them until they are not in a deficit anymore and therefor wasting the gold spent to recruit them in the first place. They also really like to sit on their ressources the whole time, not building anything up.

The second biggest problem of CK3 is the missing flavour and lack of event variety. There are barely any events in the game, resulting in often seeing the same event multiple times with 1 character. There also are barely any events tied to a specific region, resulting in everything being very blant, repetetive, uninteresting and generic wherever you play.
The balancing in general is pretty bad, e.g. some MaA unlocked in the early game are just way stronger than some unlocked in the late game. There also are a few traditions that make conquering stuff way easier with almost no drawbacks. Some events for example have 2 choices, one of which has positive effects and a minor bad effect, which ofc is a good deal. The second option however ONLY has negative effects, in some events these are even worse than the bad effect gained in the other option. This is just flat out bad game design and no one ever would pick the second one.

CK3 barely has any bugs and a decent character and CoA editor so thats a plus (?)

Crusader Kings 3 really is a sad story. The core concept is great, the execution is horrible.

I would advise not to buy this game in its current state at all, wait for it to get better (if it will ever be, guess not tbh)