Here is an example of how "not threatening" rebels can be:
The Ottomans previously owned all of Syria, Bulgaria, what is now Morea, Armenia, the Oman provinces near Egypt, Transylvania, and the Georgia area. If you look carefully you can see their land bordering PLC and down to Ethiopia. That was contiguous.
Then I fought one war with them, a war of opportunity to take revenge for them making my Tabby --> Persia run hard on me by backstabbing me while I fought a war against Timurids (that was a tough moment, loans and looting after giving stuff to Ottos kept me afloat). Their FL went from 180 to 88, and the only nation I made them actually release was Bulgaria. After their terrible run in with rebels, I annulled their treaties too.
Now keep in mind, the new rebel rules mean that this can be done to other players too. Rebels don't need to be stronger, the can already break 100's of war score free in short order.
This is ironman obviously, so lucky bonuses against rebels and whatnot are in play here too.