Hi Gavin,
From past experiences, the reason why behind is the complexity, since the parent Nexus have to be configured in a vPC domain, you will be dealing with two separates configurations, which have to match in order for the FEX to work evenly. any mismatch in the vPC peers configuration could potentially break the FEX connection. You can use the Switch Profile feature to address this, but i believe this feature is not broadly used for the problems associated with it. The high availability can be also be more easily achieved by having dual-homed Servers connecting to single-homed FEXs with Parent Switches in a vPC domain. This topology is depicted in the following link (which also contains the other possible topologies)
http://www.cisco.com/c/en/us/support/docs/switches/nexus-2000-series-fabric-extenders/200363-Nexus-2000-Fabric-Extenders-Supported-Un.html#anc10
I hope this information could clarify your doubt.
-Enrique
