Don't see why Bounty Hunters would not award xp or decent gear if of high enough level. I seem to recall several bounty hunters and assassins seeking me out in BG1 and BG2, not to mention the old 2.0 Mage Hunter specialization available to rangers.
The idea is to decentivise killing an entire town deliberately to spawn a bunch of them to kill for xp and gear. Maybe there's a better way to do that but essentially what I'm trying to highlight is that this is something to be avoided.
Being debuffed by "marked for death" or "hunted by bounty hunters" seems like a worthy debuf if it prevents fast travel and long rests for the duration.