I found some documentation that I wrote a few years ago on the Nexus product line, a few things have changed since I wrote this, but it is largely true.

Dual Homed or Active/Active Fex Design
The Nexus 7K to 2K does not support dual homed connectivity. Cisco does support this in 5K to 2K design, but not in 7K to 2K. The single homed design is the only supportable solution. This has implications that if there is a single Nexus 7K failure, it will take down all of it’s down stream FEX’s.… Read more