i used resilio for years and thought the same about syncthing until i switched last year and now I actually prefer how syncthing is setup.
if you pay for resilio there is an option to add all your folders in one go but on some computers I don't want to add all of them anyway so that's not much use to me.
with the free version you have to manually add folders one by one but to do that you need the key which means you need to copy them to a text file and add them on another computer.
with syncthing, it will detect other syncthing devices on your home network so you just have to add the ones you want then accept the request from another device.
once that is done you select which folders the device has access to and then a notification will show up on said device asking you to connect. so basically no fiddling with keys or having to store them somewhere secure
(this is all presuming i was using resilio correctly, maybe there was an easier way I was not aware of)
It's basically a P2P based Dropbox with no accounts, full end-to-end encryption and no folder size limits.
It's not open source, but it can work without a central server if you need it to. It's also amazingly simple to set up, much simpler than sync thing.