I don't know specifically how these operations advertise, but what some people might call a scam is that cryonics does factually damage your brain extensively, and it's unclear how well anyone could possibly recover from this damage. One of the common thoughts is that maybe super advanced future sci-fi level brain scans could digitize your brain, but taking anyone's money with that as a potential outcome is probably less than awesome. And there's no currently known way that we could thaw these people's brains in a way that would persist life basically at all.
From my understanding (and I never seriously considered cryonics) all patients know very well that the likelihood of recovery is very low. Just not zero.