Thanks for clarifying! I can totally see where that sort of stuff can really mess things up.
My experience with development environments has been a bit better: Node works out of the box, no problem. For Ruby, the workflow took a little setting up (with bundix), but ended up working very reliably. For R, I actually enjoy that I can set up all my packages with home-manager and they get updated in my regular update cycle and it's not a separate process altogether.
Out of curiosity, what do you mean by "pain with static linkage"? If my links have broken in NixOS, it was always due to my inability / laziness to set things up correctly.
I have been using NixOS for my daily driver for about a year now, and while it has been a bit of a learning curve to set things up and heavily rewrite my dotfiles, the dependability and availability of packages has been nothing short of amazing. It feels a lot like the final destination for my distro hopping journey.
I use a lot of CLI tools and some system level hackage to get my keybindings just right, so when I tried out Silverblue I had to load in a lot of stuff through rpm-ostree, which was less than ideal. But if OP wants a rock solid system with Flatpak apps, I wholeheartedly second Silverblue.
That seems to be a common usage of the term, but strictly speaking, "userspace" is anything that's not the kernel. This includes system-level programs, libraries and settings configured as "root" that can affect all users.