One of the main problems I have found with them was the fact that sync did not work correctly for all. Only two of them were able to sync a large folder structure (hundreds of thousands files in thousands of folders) without mysteriously duplicating, deleting or "reappearing" files, instantaneously detecting changes and replicating them without issues. It even handled things like creating, renaming and deleting files quickly, without duplicating them.
But this year, I decided changing that service for another one, that would encrypt my files before leaving my machine. Some of these services call themselves "zero-knowledge" services, because (as they claim) clear text does not leave your host: they only receive encrypted data — keys or passwords are not sent.
I did all testing I could, with the free bit of their services, and then, chose one of them. After a while, when the load got higher (more files, more folders, more GB...), my horror story began. I started experiencing sync problems of all sorts. In fact, I have payed for and tested another service and both had the same issues with sync. Worse, one of them could not even handle restoring files correctly. I had to restore from my local backup more than once and I ended up loosing files for real.
In your experience, which service (or services) are really able to handle more than a hundred files, in sync within 5+ hosts, without messing up (deleting, renaming, duplicating) files and folders?