With todays release of docker-sync 0.4.0 we probably have a promising final solution.
Since the new :cached mode wont be to effective ( see https://github.com/EugenMayer/docker-sync/wiki/4.-Performance ) we will need a external solution anyway - :cached is still 50 times slower then native.
The biggest concerns using docker-sync were host-dependencies - and that has been removed - there are no host dependencies any longer - just a gem on your system ruby. No brew, unison or unox.
In addition to that, no filewatchers are used anymore, so no crazy CPU usage. OSXFS is used in a away that the OSX->sync-container sync is done using the low level implementation of OSXFS, but the mount into your app container is done using a live-copy - that gives you native performance while having a 2-way sync without hazzles.
Just try it yourself and check the performance table https://github.com/EugenMayer/docker-sync/wiki/4.-Performance
Have a try, would be happy to have some feedback on this.