Do you have any events in the last hour ?
Do a
Get-VIEvent -Start (Get-Date).AddHours(-1) -MaxSamples ([int]::MaxValue) | Measure-Object
And then a
Get-VIEvent -Start (Get-Date).AddHours(-1) -MaxSamples ([int]::MaxValue) | where {$_.Name -match "migrate"}
Same as you did, but without the Entity parameter