Admittedly, I've never used IHG's BRG. I've, however, use the guaranteed room reservation a couple of times. The only time I was refused was because of G7 meeting taking place in the city. Other times, they may only offer very expensive rate and/or room, but I have never been refused just because it's peak season. I wonder if Marriott simply doesn't care when a hotel doesn't sell rooms on Marriott's website but make lots of room available under Bonvoy-ineligible rates on its own website?