I use the Unison file sync tool to keep backups of all my important files on flash drives and servers. For mobile devices I do use Syncthing because MTP is painfully slow and taking the SD card out of the device to plug it in is too much of a hassle, but I would rather use Unison.
With Unison syncing is a manual process, I run it and it tells me what's changed on each side and I can make changes as appropriate. Syncthing is a bit too automatic for my taste and its conflict resolution is a bit more involved.