I played a game starting with bulletproof fighters. They seemed ok, but generally not as good as regular fighters, because the AI rarely constructs basic turrets, and fighters do so little damage to other fighters in the first place. The only real upside is that they cost metal only, which is helpful if you're overloaded with metal. I think they need some kind of a tweak to make them more attractive, but I could be wrong. Interested to hear your opinions on this.