I have experienced a short wait, and been there when there were longer wait lists started. Never has happened in SFO, but I am usually there earlier in the day. I agree with your assessment of the ORD Polaris buffet - the restaurant is the way to go - but there are times that the buffet is OK too.
BTW ... I assume you were looking at the main buffet, not the one at the same end as the restaurant. The main buffet definitely is better than the UC's.