I love my Xmonad. I haven't customized it except for one thing for fullscreen windows. I have no widgets or toolbars or desktop icons or anything besides dmenu as a launcher and xterm for everything else. And I love it. However I have some subtle graphics issues like screen tearing when watching certain 4k content, hidpi scaling issues that I could never resolve for all applications and sometimes my GPU doesn't like my TV (which is my main monitor). These are likely the fault of nouveau, but I wonder if Wayland will fix them.
I really wish XMonad would support Wayland. I don't need it to, but gnome on wayland was just really really smooth. Maybe I can set up another window manager with the exact same key bindings on wayland, since like I said I don't customize it at all.
With NixOS you can upgrade your entire OS and if you don't like it roll it back like nothing ever happened. You can also replicate your entire machine by copying your configs over to another computer, running the install, and then copying over any files you have in your home folder and you will have reproduced your desktop.
You can also very easily use a different version of a packaged app by adding an override in your config. This is useful if you want bleeding-edge features or if something is broken. Also every package is also a development environment, so you don't have to worry about setting up a dev environment to hack on stuff.