Given that you believe that unfriendly AI is likely, I think one of the best arguments against cryonics is that you do not want to increase the probability of being "resurrected" by "something". But this concerns the forbidden topic, so I can't get into more details here. For hints see Iain M. Banks' novel Surface detail on why you might want to be extremely risk averse when it comes to the possibility of waking up in a world controlled by posthuman uploads.