[jkarni]

You need to get caching right if the builds aren't going to take forever, which is a bit of work, especially since smart GC is hard, and it can be slow (the nix store gets big very quickly!). Then you also would ideally provide that cache (as a nix store cache) for yourself and others, so you don't have to rebuild it. But GH Actions doesn't do that, so you need to find a way on your own, or pay for a service like Cachix. Pushing the nix cache can be quite slow. Then the logs are formatted in a very nix-unaware way, which makes it hard to find what you want. And Github doesn't yet even have M1 Mac VMs, so you would get to test/cache that. (I think it's also not possible to do other architectures, like aarch64.)

Also, for whatever reason (in addition to the specific ones above), GitHub action runners are just slow...

In short: garnix is faster, has more features, and is easier to set up.