That's a fair criticism of the general SAT - I used to teach those classes, and the test is very gameable in the sense that there really are "A FEW SIMPLE TRICKS!" to learn that have nothing to do with actually knowing the material in a useful way.
But at least some of the subject SATs are not like that at all, specifically the math/science ones. There really aren't many tricks or traps, they really are like normal school tests (if multiple choice) where doing well on them requires you to know the material they cover. Nobody who is "good at tests" is going to 800 the physics one without knowing physics well enough that they'd do well in a freshman mechanics course, and someone who gets a 400 either slept through class or is going to struggle at college level.
But at least some of the subject SATs are not like that at all, specifically the math/science ones. There really aren't many tricks or traps, they really are like normal school tests (if multiple choice) where doing well on them requires you to know the material they cover. Nobody who is "good at tests" is going to 800 the physics one without knowing physics well enough that they'd do well in a freshman mechanics course, and someone who gets a 400 either slept through class or is going to struggle at college level.