You don't need bind mounts, you can just pass access rights to directories into the sandbox directly. Also sandboxed apps run inside a (filesystem) container so file writes to $HOME are transparently redirected to a shadow home.

Respectfully, it's not enough. You can't treat the inside of the sandbox as a generic macOS system. You can't really install arbitrary things or run arbitrary programs. The wheels fall off extremely quickly.

That's true which is why I abandoned that approach, but the original comparison was against Bubblewrap which has the same issues (yes with enough overlays you can make a semi-writable system into which you can install things but you can tunnel brew outside the sandbox also).

Bubblewrap does not really have these issues at all. It's pretty much full containerization.