When migrating from a member server to a member server in the same domain, local users and local groups are migrated correctly. However the local groups are not populated properly. Only local accounts are being added.
Local groups on member servers may contain local users, domain users from the same or different domains and groups from the same or different domains.
The operator running ServerMigrator must have local administrative permissions to the source and target servers as well as administrative rights to the domain and all other domains that may have accounts in local groups inorder to resolve the SIDs of the accounts to be added to the local groups membership on the target server.
The easiest way to accomplish proper administrative access would be to add the ServerMigrator's operator account to the Domain Admins group of the domain that the source and target servers are members of and add or verify that the Domain Admins group is a member of the local Administrators group of both the target and source servers as well as the local domain Administrators group of each domain where accounts need to be resolved.