Having gone through it I think the biggest hurdle is the 12ish credit hour paradigm in undergrad. You learn so much faster in grad school because you can actually dig in and focus on one subject or really one project within that subject at once. As a TA it was pretty common to hear students didn’t study at all for my exam because they had a chemistry exam that seemed more challenging and worthy of cramming, so as bio majors they failed the bio exam to cram for a chemistry prerequisite they might not have passed either. And I can tell you from experience that whatever they toil over in the library for that chemistry class that absolutely none of that will be used in their careers in biology or medicine. You don’t need to memorize how a paladium catalyst works in any job on this earth perhaps, but you need to do that and be able to see through basically a riddle on top of that to pass your ochem requirement, along with a dozen other similarly inane synthesis exercises you forget as soon as the term ends.
And what is worse, many students fail their ochem and the weight of that on their gpa sometimes forces a pivot out of the field entirely. Another potentially good scientist lost to unhelpful process.