And it turns out you are misrepresenting the situation completely. You can run M/R over key sets in the billions of keys. It sounds like you've not organized your data at all.
You're bashing a product here based on your lack of knowledge, not the products lack of capabilities.
Based on what I've seen for some internal things, that's one claim I'd like to see support for. m/r on Riak has been an unmitigated disaster here for anything beyond incredibly trivial working sets.
I've never seen a Riak MR job over more than 3 million keys complete, on a 6-node SSD cluster. It might be possible, but you'd have to throw a lot more HW at it than the comparable Hadoop setup.
You're bashing a product here based on your lack of knowledge, not the products lack of capabilities.