OK, so like many people, I'm moving over from Sync. Not a philosophical issue -- I even bought the Pro (now that it's one-time, and not a subscription). It came down to two things:
1) Sync doesn't handle massive things very well. And as you'll see below, I have massive things to be handled.
2) At least at a large scale, reliability breaks down. I've had ghost deletions, and zombie files, and things that wouldn't sync, and all sorts of other issues. It's a headache for me, but since I'm IT for my mother and sister, it becomes untenable. Particularly because we live 13 timezones apart, and I work 90 hours a week, so it's difficult to respond to issues quickly.
I use it for a few different things, mainly the following:
1) Server syncing. One folder for torrents (both manual and automated) and one for the downloads. This goes to both my PC and my mother's PC.
2) Phone. This is just for me (goes to both my PC and my laptop), I find it easier to have a sync set up so that I just have to drop a file into my phone folder on my PC than screwing around with the FTP on the phone. Also, photo sync from the phone to the PC, and doing housecleaning on my pictures is easier on my PC than my phone.
3) Development. I do a lot of programming (c++, php) and materials creation (gimp/video mastering/av transcoding/etc) for my company, and I like to be able to seamlessly transition between my desktop (for at home) and my laptop (for in the office/on the train/etc).
4) Backups/duplication. Both my mother's and my files are backed up to each others main PCs (and my sister's are mirrored there as well). Additionally, 2 other computers in her house have her music/movie/tv collections mirrored to them so that she doesn't have to go over a network share for them. Also, my mother's laptop has everything that she wants to take with her when she travels mirrored to it so that she doesn't have to dump a bunch of stuff over every time she leaves for somewhere.
As for scale, altogether it's about 7tb and 4,000,000 individual files. The smallest is a single byte, and the largest is almost 200gb. The total number of connected devices is 1 phone, 4 desktops, 3 laptops, and 1 rackmount server. They're spread between 4 cities on 3 continents, and have Windows (7, 8, 10), Linux (Gentoo), and Android OSes.
I'm still (slowly) migrating over, but so far, it looks like it will at least match, if not beat, Sync on RAM usage, trounce it on CPU usage (after initial hashing), but lose on transfer speed.