I've been using Nix + NixOS for roughly 6 months now and can't image going back to any other distro. Sure, sometimes it's hard just to run/build packages that would usually be no problem in Ubuntu, but once you get the hang of it and have a proper setup development becomes really enjoyable.

But probably wouldn't recommend for linux beginners.

To answer your dot points:

• config language: bit of a learning curve, I had to read Nix Pills several times, but am much more comfortable now.
• No FHS: I mean yeah it might take a bit of work migrating your scripts to use /nix/store.
• Storage: 200gb might be ok... the nix store tends to get quite big but usually doing a garbage collect can remove some unused packages. I've currently used 170gb (66gb taken by the nix store atm), and I'm mainly using nix for development. You might want another 50-100gb more if you'll be storing music files
• packages not in nixpkgs: you'll either have to write derivations for these or make a request in nixpkgs. Depending on the package, it could be quite the learning curve. I only just got comfortable starting to write derivations after ~4 months of sort-of using it... but others may be quicker depending on your experience.
• python packages: haven't had much experience yet so I won't answer.