In fact there's no problem of common mode noise for a single ended input which is grounded to the ground plane. The purpose of the two-stage balun is to improve the ADC linearity by driving it with perfect symmetrical differential signal. Main disadvantage of the circuit is the relative high high pass cut-off frequency. Typical support forum question, "how can I use it for DC input"? Answer "not at all", but high frequency linearity is considerably better than DC capable true differential amplifier. Thus it's preferred to demonstrate ADC performance in AC- only applications.