Is IQ really legit? I heard form Nassim Nicholas Taleb that the math used by psychologist is weak.

It's not. It was originally used to find profoundly disabled children. But know extended to be the be all and end all of intelligence. It's weak science.

its legit in the sense that is a huge variable in predicting general life success/educational attainment.

if you think those outcomes are decent proxies for intelligence then IQ is useful