Good idea !
Allowing the player to use only one axis at the time is pretty neat.
The thing is yankee is over powered compare to the others : You can go up but also move once in the air !
The aiming is a bit complicated because you toast grenades more than firing bullets and with this kind of camera configuration it's harder. Maybe make the camera over the shoulder of the player with slower speed ? Or change the firing mechanics ? Up to you :)
Good job !