Notes from a multi-host config: what should the Nix language be able to check?

I’ve been running a small fleet from one flake for a while now: a server, a laptop and a Pi, organised as a table of hosts where each host lists the roles it plays. I love Nix and I’m not here to complain about it. I want to start a thread where I collect the places the language forced me to invent some functionality to compensate for a quirk of Nix in its current state, and ask whether other people have hit the same things. I plan to keep adding posts here as I am studying prior art, so treat t...