I do not see any of those other NoSQL databases as really being equivalent. MongoDB intends to be a general-purpose application database. It has many of the features developers expect from MySQL/Postgres, such as arbitrary numbers of indexed fields, partial record updates, aggregation queries (simpler than Map/Reduce) and many others. Couchbase may be much closer in feature-set but its developers claim they do not really compete with Mongo.
I do not see Riak or Cassandra as competing at all. In fact I would expect most applications that use Riak or Cassandra are also using a general-purpose database as well (such as MySQL or Mongo). You could use some of those databases as a general purpose database but it would be more work for little benefit. It makes more sense to me to use Riak or Cassandra for use-cases that really need high-throughput and unlimited write-scalability and use an app database for things like user accounts and preference management and all the little things that can take up a lot of development time but will never have really demanding runtime requirements (for 99.99% of internet apps).
Good points for sure. I think though I'd personally look at the different solutions on both an architectural and feature basis. A good number of the reasons that the original article listed as issues they came across, were outside the realm of features available in the actual MongoDB system (more or less) such as problems with logging, monitoring, backups, etc and were more architectural issues. To be certain, these can (and probably will) be issues with other systems to investigate.