Is it practical to measure an unknown capacitance by measuring its impedance?
There will also be effective series resistance and effective series inductance that will add to the measured impedance. If these are small compared to the capacitance, you can get some accurate measurements. 

Assuming C is large compared to ESR and ESL, the inductive component will still dominate impedance at higher frequencies. So from that regard it is probably better to use a lower frequency test signal. OTOH if the frequency is too low for the given capacitance, it'll just look like an open circuit. So you'll have the find the sweet spot for the test frequency based on your ESR, C, and ESL values.