They are not all that good at measuring achievement in specific courses other than math (regular level, not advanced level) and (somewhat) English. Better measures of high school learning would be achievement type tests like SAT subject tests for regular level courses and AP tests for advanced courses. But these seem to be out of favor as either college admission criteria or measurement of school quality (as in, if the A students in an AP course all get 1 scores on the AP test, that is a warning sign about the school).
The US seems unusual in that college admission testing is much less subject / achievement based than in other countries.