Until something happens that the average user can't recover from... Ransomware for example will end up encrypting all the files you've synced as well.
(Though I'll admit I don't know as much about the new "Backup and Sync" app, I haven't used Windows at all since that came out so I'm not sure how well it works.)
If you're a power user you can work around this sort of thing but there are still gotchas for the average Joe.