Main reason for leaving was that Nix package maintainers have to heavily patch all software. Packaging for nix is more like porting software to another operating system. Just check the amount of ad-hoc patches in nix-packages repo, and note that they have automated tools for patching the most common problems, so the problem is even worse. Now how about quality of these patches... I was able to merge some pretty large patches to nix-packages from an anonymous and sketchy-looking github account, and they weren't scrutinized much, because the original author of the derivations abandoned them.
Moreover, Nix breaks the chain of trust for the language packages. For example, Erlang and Elixir packages are signed cryptographically. Most erlang libraries come with a rebar.lock file containing hashes of the dependencies, so reproducible builds are already ensured. Unfortunately, package hashes are incompatible with Nix derivation SHA's, because they include some additional envelope data. What did Nix people do to work around this problem? They patched out the checks for rebar.lock files from the 3rd party Erlang build system. I would not dare to run a distro that contains patches like this in any kind of production environment.
Additionally, /nix directory is readable by all users, so you cannot use Nix to manage secrets, and there was no universally approved way of doing so (or at least it was the case at the time I was using it).
As for personal use... You cannot install opam packages without dealing with incompatiblities, you cannot easily install games from Gog without dealing with incompatibilities, and to be honest, do you really need such degree of reproducibility for your very own dev machine?