Do you use it for images, too? Maybe using a large file extension. Dropbox syncs around 2TB of random files successfully, and it seems like that is not something git it very good at (judging by game developers using Perforce or others to share files).
I am always open for recommendations to replace Dropbox. Even Gdrive (one of the most obvious alternatives) choked on performing the initial sync (of 2TB) last time I tried.