1) If you have 20 virtual instances on one physical host, that host becomes 20 times more important than it was before. Other things being equal (OTBE), that's 19 more phone calls in the middle of the night than the case of only one, when a physical instance goes down. And these are computers. Go down they will. True, it's only the Administrator who gets those emails or calls, but what if that person is on vacation, doesn't answer the phone... or quits?
2) A virtual instance simulates various hardware components with files representing or standing in for them. This does not eliminate everything. This means that a FEW problems go away, like power supply or hardware-related ram failures (out-of-memory problems will still happen). Other things remain, like ALL the administration issues, jobs and potential worries. ALL OF THEM. Although it's virtual, it's still a computer.
3) I can foresee a day when virtual computers will be more stable and reliable than physical ones. We aren't there yet by any measure at all. Those who rush to virtual computing due to cost alone are slowly finding out that there's no magic bullet here.
4) It's just as easy to underbuild a virtual computer as it is to underbuild a physical one.
5) And as one is dealing with these virtual machines, add in all the other problems of networked arrangements like this ("the cloud" or "the fog"), like security issues, the it-all-goes-away-if-you-don't-pay-your-bill matter, the fact that virtuality does not constitute a backup (you still have to do that yourself unless you have a managed - EXPENSIVE - solution), etc., etc.
