Syncthing is one of the "fire and forget" kind of programs. It takes some time to setup when doing for first time, but then you pretty much forget its there, and when you need it, you realise it has been chugging along doing its job.

Two things I like about it in particular, are

1. It can and favors syncing over local network. This has a huge cost savings in developing world. Even when you do have massive bandwidth, local network sync still has more throughput and lower latency. My music is shared between devices and if I add a track, it takes less than a second to appear on my other devices.

2. You can set conditions, such that delets can be ignored. Eg. I have a WhatsApp message+media backup going back almost 7 years now, ~65GiB. But I don't need all that on my phone. So I just disable syncing delets on my storage. Now my phone can get away with ~3 GiB of whatsapp data (only because I don't delete very often) and I still have complete backup in case I need it.

3. Absolute rock solid stable. At $DAYJOB we use syncthing to sync multi dozen GiBs of new data every day, total file count in 10s of thousands in about ~300 directories. And in past 5 years we have had about 3 instances where we had something that needed attention, out of which 2 were not syncthing's fault.

I've been using Syncthing for years and have absolutely zero complaints against it. Everyone with 2 or more devices should give it a try, just to see there are better options out there.