I would say that (and I suppose this is a bit of a weasel words style answer) each is going to have their benefits and drawbacks and they would have to investigate each for their particular use-case to determine which would best work for them. I think it's pretty true though. I didn't mean to imply that Riak is a go-to solution at all. It was more to give a couple 'options' to explore if you were to start a list along with MongoDB.