I've worked on a cross-platform product that deal with files on disk. I completely understand why they limit to certain filesystems and why that list is different per OS. I briefly tested out a setup where we tried to share an external drive between Linux and Windows...I think it was formatted ext3. We needed symlinks to work and that was a no-go in Windows on anything but NTFS and the inverse wouldn't work with Linux. I've also had to deal with subtle user/permission differences when moving drives between computers depending on the partition.
It seems like a situation where 95% of your users are covered by supporting a few partition types and a significant amount of time would be spent trying to do anythings more.
The hard case of syncing files between OS is one they are already handling what they deliberately stopped handling was any other filesystem on Linux other than ext4.
This actually doesn't make much sense as logically they ought to be depending on an abstract interface to the filesystem not a particular filesystem unless the filesystem in question actually lacks features required to make the software work.
This is actually so because it actually did work for years across a wide variety of filesystems. I was syncing files between one machine with ext4, one with zfs, and my android device without difficulty.
This is perceptibly to the user like websites that used to check to see if you were running internet explorer and auto fail if you were running firefox. It actually works if you fake the user agent.
Humorously you actually can work around the problem in the same fashion.
It can't completely ignore filesystem permissions even if you're not tracking them. It's obvious Dropbox isn't the right fit for a lot of people's needs here anymore--and I agree that sucks.
I can't speak to Dropbox since I don't use it for much, but even if a filesystem has those features it's not always a good idea because of a support, q/a, performance reasons, or future features. I was just trying to give my own abridged version of "mistakes programmers make about X"
It seems like a situation where 95% of your users are covered by supporting a few partition types and a significant amount of time would be spent trying to do anythings more.
It is unfortunate they dropped exFAT support.