The air between plates of tuner capacitors will become ozone and arcing may occur unless voltage remains low. Dust and other particulates can (will) get in between the plates and collect on them. These caps were never meant for audio. It takes a leap of faith to say they will do anything good for audio, and a lot of gumption to say they have a neutral sonic signature. I guess that is one man's opinion, but I don't see any references to support it. No legititmate studies have been done, no manufacturers have come forth with a premium audio capacitor using air as a dielectric.

I could see a premium cap made with a vacuum dielectric. That might be good. I'm skeptical that scavenging old radio tuners is good for audio circuits though. On the other hand, there has been universal acceptance of other types of caps for audio such as polys, papers, etc.