The learning curve is pretty steep. It's not just a new config language, it's also letting go of many preconceptions one may have (where stuff lives on the filesystem e.g.).

That's also true for software I'm packaging. Certain ecosystems just really love to load configuration files and write log files relative to the binary path which breaks in nixos because all packages live on a read-only mount (looking at you ruby, cargo from rust will also be fun).

Lastly there's just not the same number of contributors yet so you're going to have more of the small issues that get fixed and reported by the armies of users that e.g. Ubuntu has.

I'm saying all this as an incredibly happy user. I can think of few technologies that have given me as much leverage as nixos in recent years.