Kind of depends what you want to package. For projects that force you to provide dependencies yourself (e.g. most C or C++ projects), Nix packaging is very easy to use. Just slap a flake.nix together with the necessary dependencies, where to get the source from and how to build it.

Where Nix gets really difficult is with packages that reinvent their own packaging system and do dynamic downloads at compile or even runtime. Those really do not harmonize with Nix, as the Nix build process happens in isolation without network access and wants to have all dependencies specified beforehand, with checksum and all.

When it comes to languages with their own package manager it also gets a bit complicated, as while Nix does come with workarounds for all the common cases, there are generally multiple ways to do it, e.g. you can use mach-nix, pypi2nix, buildFHSUserEnv or buildPythonPackage to build Python packages and it's not always obvious which is the best approach or which will even work.

Packages that softly depend on other packages via some kind of plugin mechanism are also tricky, due to Nix packages all being isolated in their own directories. Again, which workaround works best here can be tricky, some packages require specifying all the plugins at package build time others use environment variables or other means to locate plugins.

All that said, these issues are kind of fundamental when you want to have a proper reproducible packaging system and hard to avoid. I do prefer a system that forces some cleanliness from the ground up instead of adding ever more ugly patchwork on top, but I can understand why that can be at times very frustrating.