I worked somewhere with a private cloud. It was permanently over-subscribed and the VMs were nearly unusable because of it.
Not saying it can't be done well but there's less incentive for a company whose main business isn't providing infrastructure to ensure they've provisioned enough resources to meet demand.
That's like running against cost limits in AWS. If no money is spent, you won't have the flexibility. But it's not the fault of private clouds, they can be as flexible as a public cloud.
Not saying it can't be done well but there's less incentive for a company whose main business isn't providing infrastructure to ensure they've provisioned enough resources to meet demand.