The time between stopping the VM or application on the source and resuming it on destination is called 'downtime'.
VMs that dirty a lot of memory during the pre-copy phase tend to have a larger downtime.
Too many network faults can degrade performance of applications running inside the VM.
An ideal pre-paging scheme would mask large majority of network faults, although its performance depends upon the memory access pattern of the VM's workload.
If the destination fails during live migration, pre-copy can recover the VM, whereas post-copy cannot.