Maybe you need to try it out rather than casting doubts about a product you haven't tried yet? Pre-emptively doubting claims based on your past experience with a tool Flox claims to solve seems like a poor way to go here.

For what it's worth, there are a number of options in the Nix space that really do solve for its complexity like Devbox and now Flox.

If you still use it, my best suggestion is to use a github search for 'path:.nix TERMs' when you run in to issues. It's likely someone already has that custom package, or an overlay to fix that missing compile option, or whatever else it is that you're looking for. I find reading other peoples solutions also helps with understanding and solving related problems.

I'm in no means claiming Nix is perfect, but coming from other distros to NixOS I really appreciate the ability to reboot to a new generation and then reboot back if I run in to issues. To do the same in Arch I had a bunch of 'snapper plus manual steps' to accomplish the same.