When migrating your Groups you are seeing Throttling errors in the log i.e.
00:36:20 PowerShell Result: Get Recipient Error: This operation exceeds the throttling budget for policy part 'LocalTime', policy value '3000000', Budget type: 'PowerShell'. Suggested backoff time 282234 ms.
Normal user migrations work without an issue.
When migrating Groups you need to ensure that the number of threads/MFNE workstations does not exceed your PowerShellMaxConcurrency.
So for example if you had a PowerShellMaxConcurrency of 20, 19 MFNE workstations, all running 5 threads each, the maxmimum number of workstations that could be used for Group migrations is 4. (i.e. workstations * threads needs to be less than PowerShellMaxConcurrency).