Well it depends on the motor bieng used. for three phase induction motors(infact for all ac motors) mcsa is the best option to go about. Haven't studied impedence analysis in much detail but it shouldn't be able to deal with all major fault occuring in motor. I have myself being doing work on MCSA and believe me it really works !
MCSA is the most commonly used fault diagnostic technique these days. Other known techniques include vibration analysis, thermal analysis and noise monitoring. Impedence analysis is far behind any of them. But then again MCSA is the easiest and most effective way to diagnose motor faults.