The Map view documentation does have a large warning on rewriting history (which obviously is required for a number of operations), but I agree that an in-app warning might be good too: http://forums.gitup.co/t/using-gitup-map-view/34.
In such cases, GitUp just prompts you if you want to force push.
If you decide to force push, then your updated commit message is on the remote repo as well. This is completely safe if this is your private repo, or you are pushing to a private branch. In any other situation e.g. a shared repo with a team or a public repo on GitHub, you should pretty much never force-push as this will mess up other people clones (as there are now 2 histories of the repo in a way).
Plus, as even programming data structures are increasingly immutable, I don't see why I would go change the commit message from a week ago.
I would even go so far as to say that even with a sole developer, this shouldn't be part of a workflow, as 1 developer can become 2 fairly easily.
I think you need to mention this in your homepage/video/marketing somewhere.
You're basically making it easier to mutate Git history, with all the concurrency issues that that creates. The reason why changing history in Git is hard, is because it should be hard.