Most mentoring produces encouragement. Encouragement is pleasant and changes nothing. A session is useful when the founder leaves with a decision they were avoiding.
The format that works is unglamorous: a small panel, material circulated in advance, and a single question the founder actually wants answered. Not a pitch. A pitch invites performance, and performance hides the thing worth discussing.
The common failure is answering a different, more interesting question than the one asked — usually the strategic one, when the founder needed the operational one. The second failure is confident advice drawn from a company at a different stage, in a different market, with a different cost structure. Experience transfers less than it feels like it does.
A good reviewer says which parts of their advice are grounded in evidence and which are instinct. Founders can weigh both, but only if they are told which is which.