You have a point. If you are really good, a mercenary is good. But we are much less versatile than other classes. I think most of us mercs/commandos can agree we can dispatch most classes 1 on 1 short of a guardian/juggernaut tanker. But with PvP, its almost never one on one, because we fight in groups. How often are you close to killing a foe when another player jumps to their rescue. On one or two occasions i have killed two players attacking me at once, but only when one was previously damaged and i can get a concussion missile on the other one to hold them off. We can get stagglers, but we have to become a serious dps class that can be in a serious multiplayer fight rather than picking off a lone player on the edge of a map.
only classes you can 1v1 as pyro/assault are gunslinger/snipers. Possible ops/scounrdels but only if you can bust them out of stealth before they get their jump on you and can keep your distance from them. Vengence juggs/ and their pub mirror, again only if you can get away from them after they leap on you.
Gunnery/Arsenal have more of a fighting chance if they just plant their feet and face tank anyone in 1v1 (except snipers/gunslingers, inquis and pyro/arsenal mercs/mando)