Fedora Silverblue -- a very good balance of immutable distro and user friendliness. Stability and reliability of being immutable without low-level hacking like in Nix / Guix.
I use immutable distros for the stability, and the nixOS approach isn’t for me.
You can install whatever you like using a tool called distrobox, which allows you to run containers easily.
I have an arch Linux container, and I have access to the entire AUR if I so please. I use that container to run Steam, and performance was the same as on Bazzite using the natively installed Steam.
This is not entirely true. Is there overhead? Sure. But, if the distro used for the container provides (somehow) faster or more performative packages to begin with, then running software within a fast container can be faster that running it natively on the slower host. Link to the comment in which the link to the above benchmark can be found as proof. As can be seen, the Clear Linux container performs better in 90% of the benchmarks. And, the Fedora container is only negligibly (so within margin of error) less performative than the Fedora host.
Q: Would a normal system (read: I'm not talking about Guix System or NixOS) allow you to install multiple branches/versions of the same software natively without introducing a lot of headaches?
A: No. This is literally unsupported.
Then, if using containers (or any other similar platform) allows one to breach that limitation, would it be fair to call containers (and their like) to be strictly limited/limiting in customization?