I agree with the PPs who have said it's rude to complain in these circumstances. To my mind, it's also rather a silly thing to do, because it makes the complainer look ungracious.
By way of comparison, I once worked for a company whose Director was leaving, and he very kindly invited all the 'lower-echelon' office staff out to a lunch at his own expense to say thank you. We all went, had a lovely meal and wine.
However, one of my co-workers had taken a dislike to the Director some years previously. She attended the meal, barely speaking to anyone and never to the Director, but then groused and complained to the rest of us afterwards that she hadn't wanted to go but felt she couldn't get out of it. This really wasn't true - it was a completely voluntary event and she could easily have found a polite excuse. Instead, she came along, ate and drank on his dime, and then complained about it afterwards. It really didn't make her look good.
I do realise the situations aren't exactly the same, but I think the same principle applies.