question

Yes, you can use Cloud Script for simple, server-authoritative logic, like checking the reported results from a client for cheating (before writing it to statistics). In fact, one of our users, Hamza Lazaar, posted a sample to his GitHub showing using Cloud Script with the Photon memory demo: https://community.playfab.com/questions/9573/memorydemocloudscript-question.html?childToView=10075#comment-10075.

For a competitive game, your best bet is to use a custom game server, of course. Short of that, you would want to have each user report the results and compare them, using some sort of "trust" rating. If the reports match, register the results and raise the "trust" rating for both players a bit. If they don't match, lower their trust rating. I'd use a sliding scale for that, starting at the amount you increase for a match, but increasingly negative based on how many games they've played - that way, new players can't be unduly penalized by people who occasionally cheat to try to trick your system. And even then, you'll need to monitor your community closely. Using PlayStream to log an event whenever there's a mismatch, with a quick summary of the differences, will help you with this.

Anyway, for a mismatch, you don't need to just throw out the results, then - compare the two reports, and use the results from the player with the higher trust rating, if there's a big difference in trust. You could also keep track of the number of times they had a mismatch with their opponent, along with how often that mismatch resulted in them winning the match. If you're recording all the mismatches via PlayStream, and you're collecting all events via the Event Archive, you could use that to check who all the player had mismatches with and what their history shows, to try to make a determination as to whether they're trying to be a clever cheater.