The problem is (to me) it's entirely obtuse. I can't call the function and get back some configuration - which is insane to me. You have to pass in all sorts of state, and you have a lot of difficulty producing the exact same state as your config in question would see in a real execution. Or at least i do. I even asked on several forums and the answer kept boiling down to "Well, it's just not easy. Sometimes not possible." What's the point of it being functional?
Ie yea, i can load up the Eval and call my config func - but what about the params? Well now i have to generate the params. Some of them might be easy, but some are difficult as hell - and if they differ now executing my func in the Eval is not producing the same output (or failing entirely) as it does when i run it "for real".
Nix in practice felt like all of the problems of imperative languages but wrapped in a nice functional wrapper. It was functional without any of the real benefits of functional - to me.
Eg i can't easily get the same input and pass it into a function to produce the same output. To be able to view a function as a simple slice of functionality that i can inspect, debug, etc. They have get access to the entire universe (nixpkgs/etc), a huge stdlib, etc - and you need to recreate all of that if you want to use the function.