It's not so simple. The main complaint I have heard is that programmers find it difficult to deal with eventual consistency exposed through vector clocks. It's only recently that CRDTs have been reasonably well known, and they solve this problem. Riak 2.0 includes a CRDT library but it might be too late.

Note that Cassandra doesn't actually handle eventual consistency properly, and has weird corner cases a result (e.g. it's infamous "doomstones"). As an immutable data store it works very well, particularly when you have a high write load.