Wouldn't insincere ones be harder to really convince they're doing anything wrong (in terms of actions taken, not words spoken), since they don't care whether or not they're doing it right? Insincere ones might accept criticism and then not make any changes, whereas sincere ones might fight harder against criticism but actually make real changes if convinced. There may be some usefulness in contacting cryonics organizations about criticisms against them and eliciting responses, as well as eliciting evidence to back up responses.
Not in my experience - the insincere can be convinced to fall back to a different not-necessarily-sincere position, whereas the sincere tend to take an attack on their beliefs or actions as an attack on themselves.
The apposite consideration here is Dumas' razor, "I prefer rogues to imbeciles, because rogues sometimes rest."
(In the original French, "J'aime mieux les méchants que les imbéciles, parce qu'ils se reposent." Or various other renderings, e.g. "Je préfère le méchant à l'imbécile, parce que l'imbécile ne se repose jamais&qu...
I recently found something that may be of concern to some of the readers here.
On her blog, Melody Maxim, former employee of Suspended Animation, provider of "standby services" for Cryonics Institute customers, describes several examples of gross incompetence in providing those services. Specifically, spending large amounts of money on designing and manufacturing novel perfusion equipment when cheaper, more effective devices that could be adapted to serve their purposes already existed, hiring laymen to perform difficult medical procedures who then botched them, and even finding themselves unable to get their equipment loaded onto a plane because it exceeded the weight limit.
An excerpt from one of her posts, "Why I Believe Cryonics Should Be Regulated":