A private mortgage broker represents the lender rather than the borrower. As such the private mortgage broker looks out for the interests of the lender. Instead of squeezing a loan through the gates of the lender a private mortgage broker is a gatekeeper.
However, experienced commercial brokers see little difference in practical terms. Either way, if a broker fails to spot issues before the lender does, or worse, if the loan settles before the issues are discovered, they have burned their bridges with the lender.
As with all loan broking, relationships are the key. Relationships involve building trust and understanding of what is important to the lender.