I have to pitch Resilio Sync (formerly Bittorrent Sync) here.
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.
Syncthing supports introducer devices. If you mark a device as introducer, you'll automatically add all devices the introducer knows to your list. So basically adding a new device to an existing group requires everyone to agree on one introducer, and everyone will add the new device automatically.
NAT traversal and relay servers exist too. So seems it's pretty much on par with Resilio, plus open source.
For my specific use case, there's probably no device that could be an introducer.
It's all PCs, none of them online 24/7.
Besides, setting all that up requires way more technical knowledge than most people have. Sure, I could do it I guess, but explaining Resilio configuration to complete non-techies is hard enough.
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.