I agree. That said I switched to KDE a few weeks ago and turned into a fan boy during the configuration process. I'm very happy with my current configuration (probably a mix of it actually being good and the Ikea effect). I love that you can just configure almost anything almost exactly to your liking with not all that much effort.

I'm not a huge fan of the default configuration, but the ability to completely change it (with the many panel options) is just amazing (kind of contrary to GNOME).

I think I have figured out at least part of the reason for the spacings always feeling just a bit (or very) off (although I think it's bearable with my theme):

Their Kirigami design support library has a set of pre defined spacings and sizes developers are advised to rigidly stick to. These probably break down in many contexts and might not even be chosen well in the first place.

Another reason might be that KDE doesn't have a design group that is as strong as Gnome's and KDE might not be organized top-down as much (which is probably related to financing).

I think you can fix or mitigate well a lot of the mistakes (Qt is great) by using a good Qt curve theme.