Finally got round to playing this. It's a good game. I wasn't too keen on the art style going in (hence why I've only just got round to playing), but it grows on you the further you get in.
Undertale just has that nostalgia factor that gives it the edge for me. Fallen Down is still my go-to warm up song when I sit down in front of a piano.
Sandbox has potential, but is rarely ever well realised in practice. The issue is most developers do not have the time or resources to make it good and satisfying. That's why mods for games that are already sandboxes is the better option for the time being. Even if they also have their own issues.