The relays are only used if your devices can't see each other directly; over a LAN, and in many WAN cases, it won't even be used, and if you really care you can just disable it. And bluntly, if someone had a TLS break that could work after the fact on recorded data, syncthing is the least of our worries.
> (see deprecation of TLS1.3)
The what now?
> What does "Authenticated. Every device is identified by a strong cryptographic certificate." mean if you don't have to setup a certificate authority or buy a verified SSL certificate or use LetsEncrypt or generate a public/private keypair? Would you recognise your SyncThing certificate if you saw it?
It is a public/private keypair, and if you're doing the initial setup over an untrusted network, yes you should copy-paste the fingerprint and email it to yourself or whatever.
> How well do you know the SyncThing protocol? Do you know someone in a far away country cannot find a flaw in the implementation which allows them to sync to your device, or insert themselves as a relay node?
How well do you know the dropbox/icloud/gdrive/whatever protocol? Do you know that somewhat in a far away country cannot find a flaw in the implementation which allows them to sync to your device, or insert themselves as a relay node? Also, inserting themselves as a relay would not be a security break, since "Anyone can run a relay server, and it will automatically join the relay pool and be available to Syncthing users. The current list of relays can be found at https://relays.syncthing.net/." (https://docs.syncthing.net/users/strelaysrv.html)