When I went through the questions it really felt like there were 2 ppl writing them. Some of the questions seemed ridiculously easy and doable, while others were insanely hard.
Overall though, I would say the Khan Academy questions are good practice, and a good source to write down things you might not have known before. But overall they are much harder than the questions on the real MCAT, and the style is not really the same either. Use old AAMC tests, qpacks, section banks, and the practice tests from AAMC to get a real idea of what the actual MCAT is like. Those are the most accurate