Unrest

Unrest is your number 1 enemy. Unrest causes disloyality towards you, reduces your profitability in your settlements, and if its number is high enought your faction can be thrown into civil war.

Dont forget overall and settlement unrest isnt the same.

Settlement Unrest
Settlement unrest is only it the following settlement and causes problems only there. So if your unrest in your settlement is high enough your settlement turns hostile towards you, and became your enemy.

Faction Unrest
Faction unrest is in your whole faction, if its high enough your faction will be torn into small pieces because your whole faction turns into civil war. If that happens you will have a hard time to settle things down.