Lets just take the shortcut: Who assigns the the proofing unit? The metric doesn't even matter, it is a question of who. Now, who controls that? Once you figure that out you'll realize that you are trusting parties that you either didn't know you were trusting, or you kinda knew but didn't want to think too hard about because nobody really cares about chucky cheese coin. You can't be rewarded for performing useful compute without some degree of implicit trust, which makes you dependent on the other party. You can dress it up with quorum algorithms, etc - but it only further obscures the underlying vulnerability. That is fine for a token system, but not for a cryptocurrency.