No, they’re exactly that, the enemies with titles ‘Mercenary’ or ‘Bounty Hunter’. They appear exclusively during certain contracts to oppose you, extort or take by force what you are carrying.
Here is one save during such contract. Go to the destination and you’ll see them sooner or later, then refuse to comply to face them.
Right, got it, thanks for the save game!
So basically they are mostly like Bandit Raiders, but with slightly better helmets and some of them have 2handed swords.