1. The community is extremely polarizing. Not just politically, but even from just being a general contributor. Depending on who reviews your PR, you can have a very pleasant experience to a downright demeaning one. I get that every community isn't perfect, but after years of dealing with generally the same problem people, I just quit contributing.
2. The Nix language itself is relatively simple to grasp, but for one reason or another people tend to build it up into a rat's nest that becomes nigh impossible to reverse engineer. Because of it's laziness, the error messages are generally unhelpful, and debugging is nothing but pure pain.
3. The thing that makes Nix great (reproducibility) is also it's greatest pain point. Depending on your language, your packaging experience may be simple to downright infuriating. At one point I just stopped packaging NodeJS/Typescript applications and resorted to other means (and this, of course, tended to anger the fanatics). Bottom line is that most software wasn't built with Nix's constraints in mind which means it's generally always a battle to get things packaged correctly.
4. The kingdom building has continued to be a major problem. Just watch the Nix Discourse for a week and you'll likely see the same problem space reinvented at least a few times. For whatever reason, in the Nix world, it feels like _everyone_ thinks they can do it better. This becomes a serious problem when it comes time to actually producing things in Nix. In my time using it professionally, I had to change "libraries" dozens of times due to lack of maintenance/abandonment. I swear each programming language must have a dozen different libraries for packaging, each with strange bugs and missing implementations.
I am one of the hopeful that something new and better will come out of Nix, but I have high doubts it will come from the community itself.