Careful consideration needs to be given to the underlying virtualization platform as well as to the supporting infrastructure which includes network considerations.
VDI deployments benefit from having more than one connection server to provide high availability. Many organizations use Application Delivery Controllers (ADCs) to help balance the load across the multiple connection servers in their environment. ADCs also provide a number of other capabilities such as access control, SSO, and protocol proxy.
See the F5 and Horizon reference architecture here: http://www.slideshare.net/PeterSilva1/preso-16503raavailvdi-record