The two meters I've tried to date (one cheaper pen-type, the other a higher-end unit) both appear to provide satisfactory readings from blood purifier devices(4-16hz) with square waves.
ie, the pen-meter(35USD) read 17.65Hz whereas the upper-end meter(869S) read 17.592Hz. Comparing this to an oscilloscope which measured: 17.593812Hz
That said, I might try these same meters on a frequency generator to see how well they measure higher frequencies with various wave types if not only to pick my curiosity at this stage. Though for what it's worth both meters appear to do a respectable job in displaying the running frequencies with the units I tested on.