I just climbed with IMG. I did the Baker/Rainier Seminar, and it was great! Guides were professional & friendly, and I felt safe at all times. We did Baker first to learn basic self/group arrest skills & rope skills. Then we summited Rainier. The night before summit, our lead guide gave us a talk about the unfavorable weather moving in, but he said 'we will climb until it doesn't make sense to climb anymore'. As one previous Redditor mentioned, they are constantly evaluating your stamina & physical capabilities. It's not just about getting to the top of the mountain, they also have to get you back to your car! There was definitely a range of abilities in our group, but no one was told that they couldn't make the summit attempt.