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#1020
The main issue with double booting is that the GRUB takes over for loading the systems. In case of any problems that is hard to fix and also if you want to uninstall Ubuntu, it is not obvious how to clean up the system partition.
The best protection is to take an image of the system partition before you install Ubuntu. Then you can always go back to that.
As long as things work 'as designed', there is no problem. Unfortunately that is not always the case.