Just want to add my two cents, as I've secretly been trying out all control types (including different controls, like a xbox controller), and am an accomplished infantry in top-tier matches.
If you want to be good as an infantry in matches, attack type (inverse, by keys etc) is about as relevant as... It's not relevant. You must feel comfortable, and be able to fight without thinking. Reacting to people double hitting faster than you just means you got to have natural reactions, fast enough to block a second time on reaction. Also, inverse doesn't give you a free double hit, normal directions can do exactly the same speed if you time your camera movement properly anyway.
The only time you might see any disadvantage is blocking with keys, perhaps (very rarely you find big use in being able to block well in matches). Attacking with keys can give you some weaknesses as well, but you can learn to angle the camera to move where you want anyway.
My tip:
If you're comfortable with it, go with it... It won't suddenly make you a great player to be explained how to double hit, you should learn to use your mind instead!