Resilio SyncEdit
I started using Resilio Sync in late 2023 (Wikipedia). Design is very similar to Syncthing, but has an iOS client, and it's the latter that determined the choice when I was looking at alternatives to Sync.
Update (2026): Syncthing now has a pretty sweet third-party iOS app in the form of Synctrain, and Resilio got acquired and the new owner hasn't shown much interest in continuing development, so the calculus has changed and I wouldn't recommend Resilio Sync right now.