I've pretty much had a 1.00 K/D ratio for Halo 3 and Modern Warfare 2. Very streaky.
Every FPS has It's weaknesses. Halo 2 had It's shotgun/sword campers and Snipers in the most odd postions, and now MW2 has campers. But theres killcam there for a reason. Unless their guarded by someone or It's on hardcore you should be able to kill that guy again if you go after him.
Most of the time people get nukes because they are heavily guarded or no one goes after that certain person. I've gotten close to a Nuke several times with a SCAR-H so It's possible to do with other weapons if your good/lucky enough.
Honestly the more I think about it Counterstrike was the only game I felt to be the most fair of first person shooters. I didn't care much for the other Battlefield games so I assumed Bad Company was the same thing. Name me the most balanced shooter out on the market besides Counter Strike and I'll give it a whirl: and I'll be bound to find some flaws with it/balance issues.
It's just that simple.
