Virtual desktop infrastructure is unpredictable for one simple reason: People are unpredictable.
A server in the data center is built to suit a consistent task that really only changes when something modifies the application using it. Human beings use VDI, however, and it's impossible to know how they will behave. Add in the complex technical underpinnings, and it's obvious VDI issues force IT to troubleshoot in new ways.
VDI problems generally fall under one of four categories: persona management, performance, infrastructure and platform. These issues often overlap. An infrastructure problem might easily affect performance, for example, but each issue does have its own characteristics.