devtrackers.gg
They don't get much from a single answer, but if there's a question such as "does playing X card lead to unfun games?" they could compare the distribution of results for games where X card was played with results of games where it wasn't. Or they could experiment with tweaks to the matchmaking algorithm and see if it results in better or worse games.
Those are good options - we've looked before at, say, do people playing with or against color pair X report more/lower happiness. One of my favorite views with this data is "In which events/queues are players who lose reporting the highest fun scores?"