let me first comment the warning: Some settings - e.g. the creation of an .htaccess file or changing PHP settings might in the worst case - under certain unusual circumstances - make your site unaccessible. The warning should make you aware that you should have some knowledge of what you're doing and how to revert the actions in the worst case (i.e. via backup). I added the warning mainly to say "I told you!" if someone blames guardXT for "destroying" a live site.
Important to mention: In the vast majority guardXT doesn't cause any issues, but helps making sites more secure! I'd recommend to use guardXT and run the checks. If you really have no clue what a check is about, skip it and - if you have the time - inform yourself either via google or with the manual
If you still decide to uninstall guardXT: Don't worry, just uninstall it using joomla extension manager and manually remove all jos_guardxt tables from your database.