Updating environments with Hyper-V can be more of a challenge compared to updating an environment that consists of mere physical servers. Not only the workloads need regular updating, but also the Windows servers and Hyper-V servers underneath them.
Hyper-V relies on a Parent Partition, whether you're using a Full installation of Windows Server 2008, a Server Core installation of Windows Server 2008 or the stand-alone Hyper-V Server. When you restart the Parent Partition your Child Partitions will also be paused. How to plan your maintenance window?
Updates can result in loss of functionality. Even though updates get tested thoroughly there is a chance a series or combination of updates or an incompatibility with a third party application or service hangs up your server or results in unexpected behavior. When you install any update it's hard to troubleshoot these kinds of situations: which update resulted in the situation?
Some updates address security holes and require immediate installation in some situations: The risk of breaking stuff outweighs the risk of getting compromised.