I think is easier to see this with what is their main strength: A declarative/reproducible OS setup. Is AMAZING for server deployment (like what you put in a docker file but that not mutate after it).
But I don't think will be so nice for day to day use. That is where a mutation OS is easier.
Fwiw the best summary of "should I use NixOS" is https://news.ycombinator.com/item?id=33679621
Also since things are declarative, that means things like e.g. updating bash aliases has additional friction vs the traditional way of editing the file directly.
Additional friction (not much imo), but you will actually be able to move that bash plugin that does that thing you didn’t even realize were not available in a vanilla shell.