Stealth orbitals will stop a 700 ship fleet cold if it doesn't have cloaking. I attacked a colony with 0 apparent defenses with a fleet of not quite 400 battleships - easily enough to capture an undefended colony. Lost the entire fleet to stealth orbitals because if they have it maxed, you can't see the orbitals period, and as long as there's orbitals you can't kill the colony.