The transducer is one thing, also the electronics are highly non-trivial, to the extent that it is in my opinion the most complex of the big three medical imaging techniques used.
With CT you have some large mechanical thing, pretty much straightforward X-ray source and image sensor, slight issue with interconnect between the stationary and rotating part (that only has to carry power and digital data) and bunch of software.
With MRI you have large superconducting magnet (which is expensive, but not that complex), few power amplifiers for various coils and relatively low bandwidth RF receiver and then bunch of software.
With ultrasound you have the transducer head, which is connected with specialist flexible multi core RF cable with the frontend. Frontend has to be able to both supply large (essentially RF) power to said transducer and listen back for reflected energy with as little dead time as possible and with as low SNR and wide dynamic range as possible (and typically using the same physical wire to the transducer for both Tx and Rx path), with this whole frontend being replicated few dozen times.
With CT you have some large mechanical thing, pretty much straightforward X-ray source and image sensor, slight issue with interconnect between the stationary and rotating part (that only has to carry power and digital data) and bunch of software.
With MRI you have large superconducting magnet (which is expensive, but not that complex), few power amplifiers for various coils and relatively low bandwidth RF receiver and then bunch of software.
With ultrasound you have the transducer head, which is connected with specialist flexible multi core RF cable with the frontend. Frontend has to be able to both supply large (essentially RF) power to said transducer and listen back for reflected energy with as little dead time as possible and with as low SNR and wide dynamic range as possible (and typically using the same physical wire to the transducer for both Tx and Rx path), with this whole frontend being replicated few dozen times.