I was configuring DWM, among other things, for the last 3-4 days, and every single rebuild switch caused a new generation to appear. There were too many Systemd-boot entries so they couldn't even fit on the screen and continued down to Gen 41. It's just crazy.
Edit: This post: https://feddit.uk/post/1454174 shows the rest of the boot entries
I'm generally not a big fan of switch, as you experienced, you risk a lot of generations that you don't actually need. Also, in some cases, switching isn't possible (e.g. you remove sddm and on switching, nix wants to restart all services related to the no longer existing sddm user).
My usual workflow is for smaller changes to just do nixos-rebuild test and try out things, and when it suits my needs finalize via nixos-rebuild boot. Bigger stuff I try to isolate and then do a nixos-rebuild boot only.
you're not gonna believe this, but i'm at exactly generation 1000 right now.
All about NixOS - https://nixos.org/