azzaman333
meh
They are perfectly fine as is. Most games I don't even get Physics before Infantry.
Perhaps add a chance(25% or 15%) that they blow up themseves when attacking rival units. That would be good counter enough.
Only against Riflemen and Infantry, on the assumption they spammed so many bullets in the air that one or two pierced the balloon making it crash and burn.
I my last game my opponent protected his macemen and longbowmen with airships. It's just plain stupid.