At my last gig at Stripe we had a candidate fail the lunch interview because he spent much of the time bad mouthing his current co-workers. Not a good look. Unless there is a red flag though, the lunch interviewer doesn't have to show up for the post-interview discussions.

Literally every tech interview I’ve ever had treats lunch as part of the interview. Every candidate I’ve talked to at lunch was aware that it was part of the interview.

At Google the lunch is not scored. It's a chance for the candidate to decompress a bit (the rest of the day is back-to-back) and also to sell Google culture a bit, but it's not about getting feedback.