I don't know if it's such a good idea to start by showing how easy changing commit messages is or even have this option that easily available, because after all you're changing the history and the beginner might not be aware what this really means, and beginners seem to be the target audience for this tool.
I agree. If you've already pushed to other repos with the old commit message, this can cause issues. The only time I actually change the commit message is when I'm sure that nobody else has seen it. I'm not even sure what happens when you change the local commit message, and push it again. Does it just get updated in the remote repo?
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).