I switched from Nixos to void Linux. Here's my experience so far.

I hopped from arch (2010-2019) to Nixos (2019-2023). I had my issues with it but being a functional programmer, I really liked the declarative style of configuring your OS. That was until last week. I decided to try out void Linux (musl). I'm happy with it so far.

Why did I switch?

0. Nix is extremely slow and data intensive (compared to xbps). I mean sometimes 100-1000x or more. I know it is not a fair comparison because nix is doing much more. Even for small tweaks or dependency / toolchain update it'll download/rebuild all packages. This would mean 3-10GB (or more) download on Nixos for something that is a few KB or MB on xbps.
1. Everything is noticeably slower. My system used way more CPU and Ram even during idle. CPU was at 1-3% during idle and my battery life was 2 to 3.5h. Xfce idle ram usage was 1.5 GB on Nixos. On Void it's around 0.5GB. I easily get 5-7h of battery life for my normal usage. It is 10h-12h if I am reading an ebook.

Nix disables a lot of compiler optimisations apparently for reproducibility. Maybe this is the reason?

2. Just a lot of random bugs. Firefox would sometimes leak memory and hang. I have only 8 GB of ram. WiFi reconnecting all the time randomly. No such issues so far with void.
3. Of course the abstractions and the language have a learning curve. It's harder for a beginner to package or do something which is not already exposed as an option. (This wasn't a big issue for me most of the time.)

For now, I'll enjoy the speed and simplicity of void. It has less packages compared to nix but I have flatpak if needed. So far, I had to install only Android studio with it.

My verdict is to use Nixos for servers and shared dev environments. For desktop it's probably not suitable for most.