It's a proprietary, more polished alternative that offers a freeware license for personal use.
My biggest issue with Syncthing was that there was no auto-discovery, if you had 25 or so computers across a group of friends, you needed to set up 25^2 connections, which wasn't really feasible. Not sure if this is still a thing, but Resilio solves this perfectly, you just input a secret key and computers just connect automatically. The developers also provide turn servers, which is useful in restrictive NAT scenarios. End-to-end encryption is supported, both in the free and the pro versions, although the latter gives you much more control over who can do what to the folder.
I use Resilio as a "big Dropbox" with dozens of gigabytes of data per folder, shared across a small-ish group of trusted friends. It doesn't cost us anything and works really well.
The largest folders I've seen had over a terabyte of data in them, but you probably want the pro version (with selective sync) for those.