Riak is definitely an option I'll consider, but experience has taught me one thing: most of the time it's better to stay with the most mainstream tools, such as MySQL, even if that means not using the absolute best tool. As long as it gets the job done and is "good enough".

There are better options than MySQL out there (such as Postgres and Riak probably), but when shit hits the fan in production and you need to quickly bring your servers back up, you'll be happy you chose a tool that has a lot of outstanding consultants and immense amounts of documentation. Finding help for MySQL is very easy. Help for Riak or even Postgres is much more scarse. Also, MySQL is very likely to still be around in a few years. We can't say the same for most of the new NoSQL stuff.

I tried to stay true to this as much as possible in the past and it served me well. I made this mistake with MongoDB, however.

That's fair. Riak seems interesting to me more from an almost academic perspective; as I said, I haven't used it. My go-to are MySQL and Postgres, too (though I use Redis a decent bit as a communication pipeline).