Also, have you previously explored the possibility of using the NTFS change journal, and if so, what challenges did you face? (I don't mean to come across negative; I'm genuinely interested to learn why you didn't use this a decade ago, since there must be a reason).
The biggest challenges with this kind of work (not specific to the USN journal) are in the lack of reliable cross-platform support for features which can be shimmed to look similar. The more unique the code path, the harder it is to test en masse.
It's also the case that the execution environment on Windows machines tends to be very diverse due to a long history of backwards compatibility and the ability to set complex domain policies -- Dropbox strives for a good user experience, and "go talk to IT to have them change this setting" is rarely one of those.
disclaimer: worked on sync at Dropbox; don't work on it anymore; don't have current context