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.
I agree and I do think that often times people will select a technology and think it should work like some other technology, become frustrated when it doesn't and then, in turn, blast the technology on only the merits they understand.
There are certainly reasons for using Riak, HBase, Cassandra, etc. and there are reasons for using MongoDB. It is when people seem to act confused when their hammer isn't acting like a screwdriver that we get these blog posts.