From the filesystem point of view, I don't think that Dropbox is so much concerned about you losing your data because of corruption. As, anyway they are supposed to be safe and versioned in the cloud. And I think that their app is very simple, ie like no specific 'driver' hack.

Their real costly problem is related to metadata! All different file systems support different naming, encoding or character sets for file names. Also, they all have their specificities and limitations regarding the support of extended metadata or user right info. Also some will support file change notification or not, some will have a valid file last modification value that can have different level of decimal precision, etc...

So, as Dropbox ambition is that their app can fully backup and restore your system data, supporting backuping from one and restoring to another or the combinatory of all possible for each case is a nightmare.

Just a simple example: let suppose that you have a filename with the char "ü". With unicode normalization, this can be stored as a single character or as 2 characters "u" + ".." (letter + particule). Everyone uses the first version, but Mac HFS uses the second form. The crazy thing is that if you try to save a file with the first form in HFS, it will accept but will silently convert the filename. So, let's suppose that it was Dropbox asking to restore such a filename with the letter as a single character, later when listing your files to see if they are in sync, it will see another filename (based on filename bytes) but not the one it expects. So it might want to download it again and again and again if it was not smart.