I don't entirely know what Viber does, but given how very different MongoDB and Couchbase are, I think they made a terrible choice of using MongoDB in the first place. You can't fault MongoDB for that. Viber announcing this change is more of an admission that their architects/engineers made a bad choice than it is a slight against MongoDB.

Also, I don't like MongoDB very much and almost always find another more suitable database (both SQL and NoSQL) for the projects that I have worked on.