- a language friendly as Haskell, so while fit for purpose definitively it's not well digested by most, also by various longtime NixOS users;
- an unclear direction, there are countless of "side projects" and no clear path, most are not even indexed in a wiki page so you just discover by accident interacting with someone else or after a search;
- a terrible documentation probably due to the lack of a clear direction stated above.
The biggest "mean install" is true, but it's not that much impacting in the real world, NixOS real purpose is AVOIDING containers in designing an infra, not being wrapped by them or wrapping them and true x86 zero-overhead virtualization does not exists. So far only IBM Power Systems with AiX seems to have something nearly-zero-overhead built-in in the (big)iron.
IMVO that's the main point: most people, NixOS devs included, fails to see a world different than the current one. A possible answer could be keeping up the evolution of zfs and mirror some IllumOS features so we can have light paravirtualization thanks to zones on zfs clones. But as per NixOS most people fails to see a different storage than the most common today, a relict model from the '80s (does anyone remember the infamous "zfs is a rampant layer violation" phrase?). A damn real modern system should be: a SINGLE application, yes, the OS as a framework, development environment who produce a running system live out of itself. A coupled package-manager/installer/storage, because those are effectively a unique thing so we do not need a network of symlinks or containers, we have a storage behind that simply expose needed software pieces together, also a system who manage in-memory stuff the same way. Zfs was the first step in this direction, with boot environments, clones, zones glued by the Image Package System, lacking the language for a proper system integration, unfortunately almost nobody have taken care of that. NixOS and Guix System offer another piece, the language to integrate package management, installers but they lack the storage integration to generate a unique new system model.
Rediscover IllumOS (OpenSolaris) would bridge the gap providing all needed piece to start a new kind of distro and infra management for a FLOSS world where there is no need of monsters to deploy simple infra and those simple tools could scale at monster level, killing the commercial IT model of the giants and given the humanity the desktop model, the "pioneering internet" of interconnected personal system model a new start.
The lack of independent universities, big labs, is probably the root cause but as always good things tend to happen anyway sooner or later, it's the interim the bad part.