The most obvious trade off to achieve this improvement in the input range is the extra power required to bias the complementary differential pair. The offset voltage also changes over the common mode input range which causes distortion in the cross over region. Cross over distortion is a major consideration if the amp is used as a pre-amp in an ADC application. Also, the bias current will change over the common mode input range and in bipolar amplifiers it will change direction. Using an input bias correction resistor will minimize output error.