It moves your library locations when you install it, so virtually everything that uses a Users\{Username}\{file path} instead of the library’s referenced location will break. Oblivion Remastered players recently encountered this, because the game defaults to saving in a hard path instead of a referenced path. If you have OneDrive installed, the Documents folder exists at Users\{Username}\OneDrive\Documents. But the game defaults to saving in Users\{Username}\Documents. But Steam uses the referenced library location. So when Steam tries to back up your saves to the cloud, it finds an empty saves folder.

Second, it defaults to backing up your desktop. Likely because many users just default to saving everything to their desktop. Which means you end up with a bunch of broken/duplicate shortcuts on each subsequent machine you use, because they all get cloud-imported from other computers.