The Genovese family was using front bosses (or whatever you want to call them) long before Salerno, so I'd imagine Salerno knew what he was gettnig in to. But just because he was a front boss doesn't mean he didn't weild considerable power. He was still deep in the Manhatten Rackets during the 1980s.
As for the Genovese family being the most powerful: until 2010, they had only 3 made guys flip on them. Valachi, Cafaro and Barone. All 3 were Soldiers so they didn't have a ton of info to give. Even the 2 Captains that flipped in 2010, Arillotta and Tranghese, weren't based in Massachusetts. There's no way they could know much about the Genovese's power base in NYC and Jersey. The FBI just hasn't been able to reach the Genovese family's hierachy. Other than that, like everyone said they are just so damn secretive and paranoid of the FBI watching them. It seems so logical it make you wonder why the other families don't follow the same method.
"I die outside; I die in jail. It don't matter to me," -John Franzese