I am not sure "not able to be brought down" necessarily applies in the US case. Everything I have seen from fist hand accounts suggests no one is trying because it is very tricky ground to bring down aerial objects over civillian areas. If anyone finds any direct whistleblower accounts of not being able to take them down in the US please post them here.
I have seen no heat signatures and very difficult to track, very long range and ability ot turn lights on and off when pursued. I have only seen one account of electrical interference and that was a bit dubious.