Opinions on BA&BSc

Hi all, just got accepted into McGill for an undergraduate Arts degree and I'm seriously considering taking the offer, but my aim was to jointly study Biology and Psychology. I applied for the Bachelor of Arts and Science but got rejected because I didn't fulfill the academic requirements, but I'm thinking of transferring in the first year. Skimming through this subreddit it seems like the BA&BSc isn't that popular and that people are switching out, and I just wanted to know why that was? Also, how hard is it to transfer and how long does it take?

TL;DR: Why do people not enjoy their experience with the BA&BSc? How hard is it to transfer faculties?