What is a flaw of the IQ test?

There is no "the IQ test". The most prominent ones are Stanford-Binet and Wechsler.

That, I think is the first problem. There isn't a single agreement what IQ is or how to measure it. There isn't a single one for good reasons, because they all measure slightly different things. But that means that fundamentally any single IQ scale is likely flawed. (Wechsler acknowledges this. SB sorta does as well, but hides it well)

But if we're looking for a second at Stanford Binet :

It's hard to administer. Scoring requires subjective judgment. It's sexist. It uses language and situations that don't apply to current times. It's highly verbal. The normative sample is questionable (though SB-V has gotten better)

And because I've had this discussion before: I'm not saying IQ tests are completely meaningless. Yes, there's some signal there. But it's so deeply flawed signal that building rigorous science on top of it is just hard to impossible.