Ultrasound are sound waves that throb through the liquid around the baby, they might 'not' be dangerous but they are certainly uncomfortable for the baby, any midwife will tell you that. Especially the hand held ones because they issue a continious 'noise' rather than the vibrational map type ones they use in the hospital.
thats why they say not to use them for very long, its unknown exactly what the implications on the fetus are but its clear it is not a pleasant experience for them.