At two megahertz this is 325 microns. For many applications this requirement is easily met. The only exception I can think of is tube testing where a 100 mm diameter tube would exceed this speed on the outside at 700 rpm. Although this is fast some systems can go that fast. A similar procedure can be used at other frequencies and times of flight.
Examples abound of under thickness tubing parts escaping detection because of insufficient sampling.
This is more of a problem on wall thickness units because they often do not use one pulse to make a measurement or take a relatively long time to make a measurement. This means a 10 mm diameter tube spun at 3,000 rpm will only be sampled 4 times per revolution. It is unlikely that the 4 will reliably adequate define the wall thickness. Every time the transducer is pulsed the system takes a sample of the moving part. We want this interval either smaller than the smallest flaw or smaller than the beam diameter.