Having played Mercenaries many times before here and elsewhere, may I just say that they and Bounty Hunters are COMPLETELY different groups with practically nothing in common?
Bounty Hunters are lone wolves or pack animals that hunt criminals for posted Bounties. (duh)
Mercenaries are a group of soldiers/pilots for Hire who will fight in battles for the side who pays them. They are as likely to fight for unlawfuls as for lawfuls. While they can also be hired as convoy security, etc. as BH also might do, this is the one potential overlap of which I can think.
I'm finding some misuse of these terms in some forum convos and, In fact in faction RP's.
As a Mercenary, being considered the same as a Bounty hunter would frankly be insulting, as I suspect that the BH's would consider the reverse as well.