The difference between cryonics and, for example, global warming denialism is that the former makes a claim like "it is probably a good thing to do X", while the latter makes a claim like "X is/is not true". These are completely different things!

Perhaps it is better to compare it to the anti vaccine movement. They do make a claim of the form "X is good/bad for you". Now the difference becomes about evidence. For cryonics there is little evidence either way: it has never worked, and it has never not-worked. In such a case there is little we can do beyond trying to use reason alone (always a dangerous thing) and waiting for more experiments.

On the other hand, there is a lot of evidence that vaccines work just fine. Denying that evidence is wrong.