Immune to a magic sword very effective against evil because their power is mechanical not biological

"That sword is magical, right?"

Trunks nodded. "Yeah, it really is something. It was a gift from a friend. The blade can destroy pretty much anything that's evil or wicked in some way."

"Why didn't you use it against the androids, then?" asked Goku, puzzled.

"I tried. It didn't work so well; I was lucky to escape with my life. The androids' power is mechanical, not biological. Not only is the sword useless against them, but I can't even sense their energy."

- Bringer of Death Chapter 31