It's a double-edged negative... not only do we not know how good the copy will be, we don't know how good is good enough. (Of course, if our standards for "good enough" are sufficiently low, then they can be satisfied by other people being born.)

Ultimately the cryonics argument is that the value to me of someone who meets my standards for being me existing in the future is so high that any increase, however small, in the chance of that happening has a higher expected value than anything else I could do with the resources consumed by post-mortem cryonic preservation of my brain (or at least, higher EV than many things I am currently doing with them, which I should therefore give up doing in favor of cryonics).

That argument seems to me to be based on an incredibly oversimplified view of what the recovery process would look like. It's not going to involve restoring operation to the system.

Quite. We don't know, so what are the chances?

They don't need to be very high for cryonics to be an improvement on, y'know, definitely dying.