Breakthrough Starshot is about sending a couple grams of tech on a multi decade journey. We are very far from having the tech to do this. You’re right. They don’t technically need to be FTL capable. But the rest of it remains. You’ll need an immense amount of energy move this far.

If you can get balloons into earth safely, you can also just pelt the planet with big chunks of metal at speeds more deadly than nuclear explosions.