Final answer:
The key issue with backing up VMs is that the restoration process can be time-consuming, which can be disruptive depending on the size of the VM and network bandwidth.
Step-by-step explanation:
The caveat of backing up Virtual Machines (VMs) among the options provided is that restoring can take time. When restoring a VM from a backup, especially if it is a large VM or if the restoration is being done over a network with limited bandwidth, the process can be time-consuming. The other options: backup size being unlimited, backup frequency being limited, and backups being automatic are not inherent caveats but rather factors that could vary depending on the backup solution in place.