I used to think the SATs were fair. Until I found out how much money was being spent on SAT prep. And those expensive prep courses had the potential to increase scores over 100 points. I took the recommended SAT prep course through my high school. It was 2 weekends of going over material that might be on the exam and a workbook recommended by The College Board. Imagine my surprise going to university and meeting stud…
While it's true that prep courses can help dramatically, the SAT subject tests are also not really that hard. I don't consider myself terribly smart (certainly not MIT level) and I managed to get an 800 on the Math II and 800 on Chemistry with a minimum of studying using pretty inexpensive books. This applied to many of my friends as well.
At the GRE level this is even more extreme. For competitive programs a perfect score doesn't distinguish you so taking the test can only harm you. When applying to grad school I was explicitly advised not to take the GRE subject tests.