The main difference is that it's easier to track what changed in the vendor closure rather than just waiting for the build to fail and updating the hash with the result that Nix gives you.
but how does it react when go.mod and the toml get out of sync? For us this will just break on CI and requires then a commit with the updated vendorSha to be fixed. I would expect the same workflow from this approach?
The only reason they would get out of sync is if you updated dependency versions without also running gomod2nix. The benefit here is that you can do that without the intermediate step of first building the project and updating the hash after the build fails.