On the Bomber balloons, I think the issue is you have to make them more desirable than devastators because they cost more. So they should be faster than devastators and have less weaknesses.
I would make the balloons less resistant to dragon attacks, slightly slower, and decrease the blast radius a bit.
Far as friendly fire goes, how often are your troops really in a position to get hit by it? I think FF is negligible for the most part in the game.
I respectfully disagree with your first point. Balloons are automatically more desirable than Devastators because many ground units will never be able to touch them. To further offset miniscule cost difference, Balloons are good units from the get-go and get great with single 25 recruit investment while Devastators pretty much mandate engine upgrade to get anywhere in time.
I agree about speed decrease - 170 should be enough to retain its mobility and give ground units at least a chance to run away from them. I'm fine with Balloons being tough and killy.
I think friendly fire a matter of principle for me, but still - not once had I Balloons with enhanced explosives parked right on top of my tanks/troopers and seen rushing enemies go up in flames while my guys just stood there shrugging it off. It feels like cheese to do this.