I'll be the hot take here and say, not at all. But it depends on who you ask and what their wish list was when going into KH3. I thought it was amazing! But as you can see here, many people felt that it missed the mark on many things, and neither of us are wrong. It's just a matter of where we were expecting this to go, given everything presented to us so far.
Do I think it's a perfectly executed game? No, it has things it can improve on plot-wise, but I still love the game quite a bit. To give context though, I'm also somebody that doesn't mind the convoluted plot points and honestly wouldn't even be mad if the next title became Kingdom Hearts Versus XIII or something LOL, so keep that in mind. So I'm WAY more willing to forgive the ridiculousness that occurs in this franchise more than others might, and that's perfectly okay.
Honesty though, hearing opinions on the other side of the fence, they've provided a lot of really interesting ideas in terms of where the story could've went and I can't lie, I really do wish we could've explored things more. But at the end of the day, this is the game we ended up getting and I can only hope that Final Mix should be the remedy to a lot of people's relatively soured feelings towards the game.
It's similar to Final Fantasy XV. The game came out late-November 2016 and while many people enjoyed it, it left a LOT to be desired (literally). You could argue that it's the worst in the franchise, or one of the best. It really comes down to what you like and whether it hits all those points for you. Sure, DLC shouldn't be used as a way to "fix" something that should've been "full" at release, but it's way better than leaving it as-is and never seeing any fleshed out changes and improvements.
At the end of the day though, what's important is your personal experience with the game. You're allowed to have any opinions and praises/criticisms you want about it, as long as you can also acknowledge that your opinion isn't law and that your mileage may vary.