My iCloud backup, with years of data, including apps which only had local storage, was completely borked when I upgraded to iOS 13 (on my iPhone XS). My phone would enter a boot loop during the iCloud restore process, and I tried 15 different restores (different iOS 13 versions including beta versions, iTunes backups, a couple different types of restores they have at the Genius Bar, a direct phone-to-phone restore, curating my iCloud backup with only the most important apps, etc). None of them worked, all ended in a boot loop. Apple Support basically gave up on me, telling me my "backup was corrupted".
In the end I had to start with a fresh iOS install, which was very disappointing, as my years of accumulated app data were lost. I manually backed up what I could, but some apps had their data lost forever, and I wasted many hours of time.
Very disappointing.