Debian

Stable/unchanging (zero maintenance for several years, no new bugs appearing out of nowhere), as minimal or bloated as you want it to be (made my own custom live/installer ISO which means I can start fresh with my environment already set up in a matter of minutes), huge number of pre-packaged software, good documentation, identical base distribution across my servers and desktops, mostly sane defaults, community-backed (no or few corporate interests driving the project to shit... look what it did to Ubuntu/CentOS...).

Other Debian-based distributions don't bring anything of value to the table for me, I'd rather use OG Debian.

These (Mint, Zorin, etc) and Debian are basically the same. You can use either of them and have mostly the same experience once you finish setting up your system. Exception here goes for Ubuntu that forces Snap. I personally go with a minimal installation of Debian and add what I need along the way. If you don't want to set things up thoroughly, you should use a distro you think has the most functionality out of the box.

I used Debian on my desktop and laptop for many years, eventually I felt like I was spending too much time thinkering. I switched to Ubuntu 20.04 LTS and everything works great. But the snap crap is getting in to my nerves so I'll probably switch back to Debian next time.

I use Debian as a desktop both for work and personal use because I am a boring person that doesn't need any surprises. I have been using it since woody after moving from Mandrake Linux

Stability

And neither of this distros are good with stability tbh

I used Debian years ago, but switched to Ubuntu. After using that for many years, I got frustrated with Canonical forcing snaps and switched back to Debian a few months ago.

I typically install my own window manager, so Debian tends to to be perfectly fine for a base install for me.

Debian testing/sid is a wonderful desktop environment if you do lots of development work. I currently use stable on my laptop, as I would rather have a stable environment that sometimes needs a backport vs. a very-occasionally-unstable environment that always has the latest of everything. But for years and years I used sid as my main environment and it was extremely convenient to have always-up-to-date development everything with very little effort.

IDK, maybe it's less of an issue now, as pyenv / npm / etc are more common and give the same type of experience. But for me back when I did it was great.

What makes Debian unfit for Desktop use? Nothing. Debian is as good a Desktop OS as Mint, Ubuntu, Zorin and any other distro out there. It isn't just for tinkerers. Perhaps it doesn't have the amount of new user friendly features but I would rather use an OS that pushes me to learn than pushes me to be complacent and also pushes me to seek the 1-click options rather than spending 5 minutes to set something properly. Debian might not be everyone's cup of tea... but for those who it is their cup of tea Debian will never disappoint.

Imo, why ask the question with the assumption "is based on a server-oriented distro" when "dektop distros" such as Ubuntu, Linux mint Zorin are then using debian? It is a bit conflicting isn't it? it all boils down to personal taste, if you like ubuntu, use ubuntu, if you like linux mint, go use that. If you want to use debian or arch or fedora, you know...

For me it is easy enough, stable enough, bleeding edge enough (testing/sid) to tinker around without invading my machine with stuff I don't necessary want

I don't see Debian as a "server only" distro.

It is good at being a headless SSH-only infrastructure server, but it's also got a huge package set and many well-supported desktop environments, so it's a good desktop distro as well.

Ubuntu-based stuff is getting increasingly full of snap and ESM advertising and other rubbish I don't want.

Sure Debian might end up with older packages later in the release cycle, but it's not much different to using Ubuntu LTS as desktop, and anyway this is largely solved these days by desktop container tech (Flatpak, Podman Toolbox, distrobox, etc).

I use a simple tiling window manager, a terminal emulator and a couple of browsers.

Why not debian?

I used to use it only on servers, but since using Flatpak, I've installed it on my media server and my desktop and I couldn't be happier.

The stable Debian base and the updates via Flatpak are the best combination.

Well,

For me its simple: i use Debian for servers, so who not for Desktops either. And personally i dont consider Debian a "server oriented distro" perse. for me its basicly a kernel with userspace software. You can make it a server if you leave it after a minimal install, or you can make it a desktop if you install a desktop environment.

Because I'm a junior sysadmin. I need to learn all the stuff as fast as I can.

Because it's the only one I know. :-<

I use it for both. It's rock solid and once setup and customised to my liking it's low maintenance. Basically I wanted a stable Ubuntu. I wouldn't say it's a server orientated distro, it's good at both.