Sometimes, backup at a hypervisor level is not possible because of virtualization product or Acronis Backup limitations.
Agent for VMware cannot back up fault tolerant virtual machines.
Agent for VMware cannot back up Raw Device Mapping (RDM) disks in physical compatibility mode when the virtual machine is online.
Microsoft Hyper-V does not provide control over pass-through disks to a host system. As a result, the Microsoft Software Shadow Copy provider cannot provide Agent for Hyper-V with snapshots of pass-through disks.