The SAT subject tests just felt like a joke to me anyways. My high school was a public high school in Singapore, a country generally considered to have great education. With a generally good mastery of the normal high school curriculum, these tests were considered very easy. Pretty much all my friends and I got 800 out of 800 for all the subject tests. And these perfect scores ended up not mattering that much in college admissions anyways.
The normal SAT I tests, on the other hand, seemed to require more critical thinking, higher reading comprehension and reasoning skills, skills that are sorely needed in an age of blatant misinformation. These are much harder to score well, which is why so many people spend a lot studying for them. Not so much for the SAT subject tests.
It depends heavily on the SAT subject test. Math II, for instance, is heavily curved and anyone with a solid understanding of high school math and their calculator should have no problem getting an 800. But I'm not sure if the same is true for all of the tests in the humanities and social sciences.
The normal SAT I tests, on the other hand, seemed to require more critical thinking, higher reading comprehension and reasoning skills, skills that are sorely needed in an age of blatant misinformation. These are much harder to score well, which is why so many people spend a lot studying for them. Not so much for the SAT subject tests.