Doesn't cryonics (and subsequent rebooting of a person) seem obviously too difficult? People can't keep cars running indefinitely, wouldn't keeping a particular consciousness running be much harder?

I hinted at this in another discussion and got downvoted, but it seems obvious to me that the brain is the most complex machine around, so wouldn't it be tough to fix? Or does it all hinge on the "foom" idea where every problem is essentially trivial?