vSphere previously implemented Enhanced vMotion Compatibility (EVC) as a cluster-wide attribute because, at the cluster-wide level, you can make certain assumptions about migrating a VM (for example, even if the processor is not the same across all ESXi hosts, EVC still works). However, this policy can cause problems when you try to migrate across vCenter hosts or vSphere clusters. By implementing per-VM EVC, the EVC mode becomes an attribute of the VM rather than the specific processor generation it happens to be booted on in the cluster.
Per-VM EVC allows you the granularity of enabling EVC for particular VMs, rather than for a cluster of hosts. This improves the mobility of a VM beyond a cluster with compatibility for returning. It also improves compatibility for cross-VC migrations without complicated stretch-cluster concepts.