As a near-fanatic NixOS user (now!), I agree with both you and GP. The learning curve is brutal, so it took me a while to get started, but it's paid off several times over.
It's unfortunate that humans can't trust each other enough that saying "You'll like it after a few months" is good enough, but it's also an inevitable fact of life; for starters, the speaker might be mistaken. I don't think NixOS is for everyone, at least not yet.
And it's hard enough to run arbitrary software that, yes, you do need to learn the Nix language. In most cases it's as easy or easier to package the application for NixOS than it is to run it without packaging, which on the one hand is a benefit to the Nix ecosystem, but on the other hand can be a little annoying. There are workarounds...
But the workarounds require some unusual understanding to begin with.
I think one of NixOS' major flaws at the moment is documentation. There's the wiki, but it's both somewhat outdated and very much not comprehensive. As for Nix, it's been best explained in a series of blog posts, and as good as the Nix Pills series is, you have to find it before you can read it. It's also somewhat short on HOWTOs.
Most people don't like to spend hours writing documentation, though.
The existence of NixOS, and work done on it, is a massive positive externality. Even if it doesn't catch on, it's going to inspire half a dozen clones in the next decade or two. Unfortunately I don't see a good way to get more effort put into it, especially the boring bits (such as documentation)...
I suppose this comment is my way of trying to change that.