So if you watch the Revision3 video where they talk about the stability issues Kevin throws Cassandra out there nothing really specific but when discussing downtime its the only technical factor he mentions.
He calls it "still beta software" and states they were fixing bugs in Cassandra during the days leading up to the release of v4.
In reply to you and epoxy, sounds like a gross failure of engineering management. If you're having problems of this sort of foundational nature a few days before the planned release, it strongly suggests you should delay and figure out what's generally gone wrong with the project.
He calls it "still beta software" and states they were fixing bugs in Cassandra during the days leading up to the release of v4.