It may not be a fad, but there's a lot of rose-colored hype that masks a world of tedium. New prospective users should be prepared for a lot of uphill grind with respect to configuring packages or defining their own packages. Every time I try to do seemingly simple tasks in Nix, I end up spending hours on the Nix discord channel trying to work out a solution with seasoned users (who invariably struggle to work out a reasonable solution). Often times I end up having to try to figure out how to package some obscure C dependency deep in the dependency tree with its own bespoke build system and so on. Nixpkgs is also pretty poorly organized (or at least I can almost never find the package I need) and this is pretty critical because something as simple as answering "what type does this function take?" requires a ton of searching around Nixpkgs (largely due to the lack of type annotations or even informal documentation). The CLI tool surface is also not particularly intuitive, and there are different tools for different problems and no consensus about which tools to use (or at least the official docs often disagree with popular opinions in the community). You have to be aware of whether you're just using the Nix package manager or whether you're using NixOS because it will affect the way you write your configuration and often snippets on the Internet implicitly assume one or the other such that you will spend a lot of time trying to fix snippets or find the right snippet for your use case. MacOS is still a second-class citizen in the Nix world and a fair amount of stuff doesn't work properly (although I can't think of specific examples off hand). The Nix expression language seems unnecessarily unfamiliar to mainstream programming languages--I don't object to the functional nature, but I still spend quite a lot of time consciously thinking about syntax even though I've been using Nix on and off for ~7 years now (compared with e.g. typescript which I've barely touched and yet is trivial to read). Normally I don't complain about syntax, but you can do very little in the Nix world without reading and writing a bunch of nixlang and even if the other problems were resolved this alone would make it difficult to sell in most organizations since every developer would have to figure out how to work with it (package management is high-touch).
I'm a big believer in Nix's high-level approach to declarative systems, but the whole UX of Nix is so poor that foregoing declarative systems altogether is quite a lot less painful than using Nix in my experience. I'm rooting for it, but it really feels like Nix needs a product manager or something (no disrespect to the maintainers; these are difficult problems and I'm sure I wouldn't do a very good job). New users should beware.