Linux

my daily driver for some years now is Debian, and I have a decent setup via Ansible - everything just works for me.

Then don't change anything.
Only because there's new shiny stuff every two weeks out there in the FOSS world, doesn't mean "old" stuff isn't relevant anymore.
If Debian suits you, keep it.

My question is mostly to long term Linux users, which use Linux in a professional context

I sadly can't speak for that exactly.
I don't use Linux professionally and would rather consider myself as "noob", but maybe my input has still some value for you.

What is your experience? How did your workflows change on your immutable Linux distribution? Did you try immutable and went back to a more traditional distribution - why? How long are you running the immutable distribution and what issues and perks did you run into?

I used Silverblue now for a few months.

I broke every distro out there and SB is rock solid. If I break something (which basically never happened), I can just reboot and select the image from before. It's a huge gain of peace of mind for me to know I never have to worry about the state of my system again!

My workflow changed to using containers for everything. As I said, I'm pretty much a "casual" user, so Flatpaks cover 99% of my needs. If I need some CLI program or something not available on Flathub, I use Distrobox, which gives me access to every distro and integrates perfectly.

Issues: nothing major yet. If a containered program shouldn't work, I can always install it natively per rpm-OSTree. Reboots aren't an issue too. I shut down my PC anyway, so I don't care. Updates get installed and staged in the background, and I boot into a new image everyday.
I never get forced to reboot, even less than on normal Fedora.

Usually, on a mutable system, you should offline-install and reboot your PC anyway for safety and better stability on the runtime.

The only thing that didn't work for me are VPN clients, but the integrated Gnome solution works fine for me too. Oh, and many install-scripts don't work/ aren't available for SB aswell.
Most might work, but nobody took time to write one, since they need some other approach than Debian or Fedora.

I really recommend you to check out uBlue, it's a great project and really "the future". It uses the rebase-feature, which enables users to make their own custom images, similar to Nix.
I for example use uBlue-Silverblue, which comes with some QOL-stuff pre-enabled. You can also install a SteamOS clone, images with integrated Nvidia-drivers, "unsupported" DE- and WM-spins, and so on there. With one command. And you can swap out the base anytime you want on an existing system.

Would I recommend you SB or any other immutable distro? Theoretically, 100% yes! Practically, in your case, no.

Stick with Debian if it fits you. Look into Distrobox if you want. See, if most stuff is in your home-directory, or if you prefer uncontainered stuff.
If the pros outweigh the cons for you, then install it the next time when you have to anyway (new hard drive, etc.).
But you can also wait a few years until immutable OSs get more widespread and mature.

Edit: I just looked up what Ansible is. In that case, NixOS would be fabulous! uBlue is relatively new and probably not as mature. If you like to install a system reproducible, just share the nix-config and apply it on another PC. But you have to get into it first, which might be complicted and time consuming. It still should be worth it.