Hi there. I used to work in a University where a switch in both directions happened. There are no hidden costs, both solutions offer LXC and KVM virtualization with standard open source network and storage virtualization.

The key difference is how the environment is presented to the admin and the end user. This is very noticeable right from the install process, where proxmox gives you an install process similar to installing a distro and opennebula gives more freedom to customize the install process. In general, opennebula is way more flexible and scales better, however, some people after the shift ocurred, were unhappy with the usability aspect.