SQL server is not good at scale-out (in particular), or DR, 2012 always on is getting there but its still complex.

This is because SQL is so tied to the servername and metadata is held outside the database in system databases.

A simple database engine only instance is OK but as soon as you start adding SSIS, SSRS, linked servers etc it gets messy quickly to maintain failover servers.

MS tend to gloss over this.