VDI planning is tough. Users don't always adopt the technology as quickly as IT anticipates, or they might use it differently than expected. In a traditional data center, those inconsistencies can cause serious headaches for IT, which has to make sure all the right resources are in place to deliver a strong user experience.
With HCI, IT can simply add or remove a node from the hardware if it needs to deliver more or fewer virtual desktops at a given time, making VDI planning easy. One problem with this approach is that sometimes IT just needs more of a single resource. For example, they could be at their limit when it comes to storage but still have plenty of compute, network and server resources. Adding an entire node to hyper-converged infrastructure means IT pays for all those resources.
Some vendors try to combat this problem. SimpliVity has nodes for individual resources that can snap on to an existing appliance. Nutanix has a VDI Assurance program where organizations tell the vendor the number of virtual desktops they need and Nutanix takes care of the planning to make sure the company's desktops perform up to snuff.