IMHO mercs are exactly what the name implies: Troops for hire.I'll look into that and I'd be in it for the money and training. I'd look into what I'd be doing before I just jump in into a group.
You have no saying -and you will be used to the advantage of shady politics -and business.
I would not risk my life defending an oil well for greedy azzholes.
Personally, I liked the military -but I deserted (defected from east Germany to the West), when the politics went wrong and I was blackmailed into stuff I did not want to do. I couldn't fight for the wrong reasons.
There are noble causes, like I said, defending endangered species, for instance -but no idea if a foreigner could join, language would be mayor setback.