These are marked beta, and they don't address all of the needs of stateful applications. This blog post [0] from the official k8s blog covers many of the caveats well, and basically describes databases as the anti-use case with statements like:

>What is the benefit you hope to gain by running your application in a StatefulSet? [...] If you have a few instances of your storage application, and they are successfully meeting the demands of your organization, and those demands are not rapidly increasing, you’re already at a local optimum.

[0] http://blog.kubernetes.io/2016/12/statefulset-run-scale-stat...