The paper compares the requirements for vibration-based condition monitoring system for two seemingly similar rotary machines, namely horizontal and vertical-axis wind turbines. The paper shows that the differences in the design have a significant influence on both, operational parameters and wind energy conversion, and what are the consequences from condition monitoring point-of-view. In addition, the authors present a list of obstacles during worldwide development of vibration-based guidelines, which has led to major constraints within practical data acquisition and processing. Finally, the paper presents comparison of experimental data and complete schemes of mechanical components of drive trains of both types of turbines.