You couldn't be more wrong. Hank has proven repeatedly to be exceedingly competent at his job, often pushing a hunch through in spite of opposition from the rest of the DEA and being proven correct each time. He is an absolutely amazing officer who just happens to have a massive blind spot where family is concerned. Walt took advantage of that blind spot, and could have gotten away indefinitely if Hank didn't stumble on irrefutable evidence. Implying Hank is terrible at his job because he didn't suspect his brother-in-law earlier is insanity.