SyncThing has been completely reliable for the past two years across my Android phone, Linux laptop, and a Linux NAS.

I use it to backup photos / videos that I capture on my phone, as well as to sync my KeePass database and Org Mode files which I edit on both my phone and laptop, often concurrently. My largest folder is 46 GB across 9,416 files.

The SyncThing Usage Data dashboard at https://data.syncthing.net/ has really interesting statistics: apparently at the 50th percentile, users are syncing more than 5,300 files and over 12.3 GB of data.