Are u/s guided core needle biopsies accurate for suspicious breast areas? how often do they miss and get normal tissue instead of the area of concern?

Yes: They are very accurate in hands of a competent breast radiologist. If there is a discrepant result between the pathologist's finding and the radiologists impression, it should likely be rebiopsied or at the very least closely followed.

Very accurate: Interpretation of an u/s guided core biopsy of breast requires close interaction between the radiologist obtaining the biopsy and the pathologist interpreting the biopsy. It is the responsibility of the radiologist to determine whether the pathology result is concordant with what is seen on imaging. When the two are concordant, the procedure is very accurate.

They are: 97%-99% accurate in experienced hands.