I don't neccesarily agree with that, since if you limited the threat to a Jedi/Sith to other Jedi/Sith then they become just a bit too powerful.
Sorry for the double post, but this!
If you want only Jedi/Sith to be threats to Jedi/Sith, then they're
the Mary Sues, not the Mandalorians. The Echani and the Teras Kasi can fight the Jedi and win - why aren't they Mary Sues? Huh? The Mandos are skilled fighters who just happen to have the advantage of lightsaber-proof armor - which, by the way, can only take so many
lightsaber blows, so it's not invincible.
But again - haters gonna hate.