I'm using Debian Testing with KDE, after using Ubuntu with KDE, and it's basically just some lack of polish, and I also sometimes miss the PPAs. (I understand that it's technically possible to use software from Ubuntu PPAs in Debian, but as I'm getting older, I prefer officially supported ways of doing things, so that I can stay on the happy path as much as possible.)
So all in all, not much of a hurt, for me at least. My home server integrations, my software and my games work just as well.
I think that Debian is lovely, but as a person with a hojillion scripts and configs, I don't know how Debian and Ubuntu have diverged over the years in terms of filenames, paths, package names. etc., and I'll have to find out (for example, Ubuntu's /etc/cloud/cloud.cfg.d/ vs Debian's /etc/network/interfaces).
I was also going to mention ufw, but I see Debian ported it, so that's one less concern!
Debian is my choice too! I run Debian testing and it is great! I have up to date packages, a ‘rolling’ release and now even Firefox straight from Mozilla
So all in all, not much of a hurt, for me at least. My home server integrations, my software and my games work just as well.