'Looking good' is pretty subjective. While a lot of these datastores include some sort of "oh, and you can use it this other way", and their marketing documents

'Looking good' is pretty subjective. While a lot of these datastores include some sort of "oh, and you can use it this other way", and their marketing documents make it sound like it is totally suitable, the point of these tests is to see what purpose you can put these datastores to and have them be a proper fit. While some are basically "you can't use this unless you really don't care about your data", the two you list are actually great examples of "provided you use them in their niche, they're great; otherwise, they're not". Cassandra, use when writing high volumes of AP immutable data. Riak, use with CRDTs for AP mutable data.