Although migrating Active Directory with a trust is ideal, there are times where this is not possible. Trusts make it possible to resolve objects’ security identifiers (SIDs), which in turn helps to distinguish objects and check that everything is going right. Trusts also help provide co-existence of two environments, including uninterrupted access to the resources for both switched users and users not yet switched.
If trusts are not established between Domains the following restrictions apply:
For a successful migration, the computer on which Migration Manager is installed must belong to the target domain.You can check out this documentation for what steps are required for configuring Migration Manager for AD to a trustless migration