It's not so much that one is forced to use Partition for so long for me, that's fine. But the way it works and, most importantly, the way it's explained makes the whole thing a whole lot more frustrating than it has to be.
The things I ran into were:
1. Claim distribution, and why it makes letting the realm split a better idea than keeping it together: If multiple top-tier titles are split, your primary heir gets claims on them, just as their siblings get claims on yours. But if you keep the realm together, you have a realm where everyone has claims on your titles, and you don't have claims on any of theirs. I really think this should be explained in a tooltip, especially to new players.
2. Illogical Dejure distribution: I haven't run into my realm capital being split off from my primary heir, but the way in which heirs get land in each other's de jure duchy (and especially in mine) bug the crap out of me. Not sure if that changed so much, because in my latest campaign, I was trying out tribal starts, which leads to:
3. Uncontrollable children: This is mostly from experimenting with some of the few flavors of paganism that offer gender equality. I honestly think it's objectively hard mode. Not will you always have more kids than titles (unless you do the things that the game is explicitly incentivizing you not to do through prestige penalties, etc.), but after a little while it becomes nearly impossible to manage their breeding. Led me to a situation in which my primary heir ended up being a landed granddaughter of somebody that didn't marry matrilineally.
I feel a lot of this is more a problem of signifying rather than broken game mechanics, as such. As to whether or not it's actually fun, well... I'd be more chill about it if I was certain that letting the game play out this way was not going to result in being steamrolled by my neighbors. Because the thing with grand strategy is, with most games, either you're snowballing, or the AI is, and its you or them, ultimately. Almost every player has that paranoia at the back of their mind, that if they're not keeping on top of things, with every advantage and trick in the book, they're dead meat.
I haven't played enough CK3 to see how justified this fear would be for this game. I have a sense that its less so than most, since the AI is dealing with the same issues for about as long as the player, but I can definitely say that for the past 5 generations in my west Africa game, I've had to deal with a Mega-Ghana on my doorstep that doesn't splinter nearly as much as mine (it doesn't help that their vassals are inheriting my land, because my female vassal members of my dynasty didn't marry matrilineally). That might be special circumstances, but the main point is, does the game actively incentivize me (through the prestige and renown mechanics) to play in a way that is of long-term detriment, or is my paranoia ruining my fun?
Also, can I just say that I miss the way CK2 handled Elective Gavekind? Sure, realm split, but at least I got to play as the best of the possible splittees with a little work. Actually, I wouldn't mind that being the first tech-locked succession type (because I have no idea why one of the cultural innovations is Gavelkind, which enables you to select a law which everyone is already starting as/locked to).
There was also the option to "duel for claims", which I almost always pursued/accepted, since I got to play as the winner either way. Miss that, too.
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