On our platform the only problem has been having to perform count operations, the way we do it scores can be listed in unpredictable fashions and MongoDB is inherently bad at counts.
Aside from that the read:write ratio massively favors reading for us and caching makes that a negligible operation most of the time, and (unless like in our case you're providing leaderboards for games you don't control) MySQL and Memcache should carry you fine.
Yes, but redis is just so nice for certain applications that can be a pain in a RDBMS. Not that it can't be done, but the simplicity and performance of redis atomic counter increment/decrement is often enough for magical vertical scaling sauce. Redis is chock full of these little use cases, even when you're just using it as a "cache".
I've got one set up in MySQL and I'm pretty scared of how it'll cope if we get a surge in usage, and that's with Memcache sat in front of it.