The main problem with Dropbox sync for me is that it often tries to ‘upload’ changes from my mobile to my Dropbox copy. It’s what it’s supposed to do, I get it.
The problem is that my mobile copy never has new work in it, only an accidental brush here or there where it’s recorded “sgsf”, which is then deleted - but now the file thinks it’s the most up to date version. 
I work on the laptop at home (not the Dropbox copy thank god, I learned that lesson the hard way) , copy it in to Dropbox but the next time I try to sync my mobile, the poor system can only see “conflicts” that it needs to “resolve”. It’s an impressive thing to attempt but I often find it loses me work.
Having the option to set sync to be one way only (download sync) would be a godsend. So long as it’s clearly labelled and is opt-in, I can’t see it causing anyone else any problems.
I don’t know if it’s possible to do within Scrivener. One for the wish list!