When a user within a security group with permission to delete constituent accounts attempts to merge two constituent accounts together, they receive the message "Important Notice! You are not authorized to perform the operation you requested." and is not allowed to complete the merge.
If one of the constituent accounts has transactions with linked soft credits, and the security group does not have permission to delete transactions, then the user will not be able to complete the merge. Admin users, or users assigned to security groups with permission to delete both accounts and transactions, will have to perform the merge.