Check the target types on the planetary warhead, it won't harm a ship. Just another case of stupid AI.
Almost any escort design you can come up with can be done with a drone. The drone would be cheaper and more importantly you can build more than one at a time.
The downside is limited movement capabilities and they blow up when they run out of gas.
I've found that they work well for early game defense. Mid to late game you just have to be more creative with them.
