Solid state batteries are also not safe. No battery is. By its very nature, if you puncture the battery, you short out the cells, current will flow, which will

Solid state batteries are also not safe. No battery is. By its very nature, if you puncture the battery, you short out the cells, current will flow, which will get turned into heat. Even if the battery is not flammable, something in its vicinity almost certainly is (plastic, rubber etc.). Especially in a house.