I'm an FPS gamer primarily and have been so since I was an adolescent (didn't have a chance as a child). I'm right hand dominant and I lack coordination in my thumbs as do most humans because thumbs were evolutionarily designed for grasping objects. I type at 120 WPM though after an injury it reaches only 80 WPM.
As you can see, my right arm and fingers are dexterous due to training. Haha, it is training regardless if you enjoy it or not. So, I play with an arcade stick with my right arm and the buttons with my left hand. I tested both ways and I'm still better playing this way. As I said, I'm a peculiar person in this fighting game scene.