This is true. The ability to invade is the only thing making this new species dangerous to empires. The fact that you cannot counterattack makes them invulnerable at the same time, which can be a huge problem depending on the size and frequency of their attacks.
We will see in a couple of month how bad they really are. They could be anywhere on the scale between nuisance and devastating.
You cannot invade their homeworld, but I believe Steve mentioned somewhere that they still have to replace their lost ships. So if you engage their ships and destroy them, intensity of their attack should be limited for some time.
I wonder, if you let them raid a lot of your ships and colonies, if they will get bigger boost and will be more dangerous.