I [wrote about this in depth][1] but here's the summary:

- Preventing split brain (electing more than one master node) needs to be solved. Failure to do so can be catastrophic
           
- There are no production ready shared storage solutions to enable databases to be shutdown on one instance and brought up on another without losing all your data.

        
  [1]: http://patrobinson.github.io/2016/11/07/thou-shalt-not-run-a-database-inside-a-container/