I havent taken an iq test. But i don't know how a formal test cannot be hacked, and knowledge tested is about structure of the test and not contents of the test

It depends on whether you mean fluid or crystalized IQ. Fluid IQ is a good measurement of ones ability in regards to problem solving and critical thinking. Crystalized IQ is more a gauge of experience and acquired knowledge. A lot of IQ tests are an amalgam of both, though some focus on one or another.

Many studies have indicated that they are prejudiced toward the majority discriminating the minority that may have a different level of experiential knowledge based on unique social/cultural backgrounds. I guess what I am saying is: they don't account for all possible variances. I like the following study because it accounts for a variance that may be considered personality characteristic, self-discipline: [citeseerx.ist.psu.edu]

Yes, they don't cover everything but they do give a general idea of ones ability to reason and understand.