The Cosmic Sword Technique was powerful in three ways.
Firstly, it could make use of the power in nature. The creator of the technique didn't have a full understanding of the power of nature, but it was already miraculous enough for ordinary people.
Secondly, it was unstoppable. In fact, Han Fei had already comprehended the idea of full offense and no defense when he learned the Sacrificing Punch. He also had the Art of Invincibility. So, it wasn't hard for Han Fei.
Lastly, the Cosmic Sword Technique's feature was speed, which was something that all swordsmen cared about.
Wuming's sword, for example, was faster than Han Fei even though he was carrying out the 64-Dimensional Fish Dragon Dance.
The same was true for Zhang Xuanyu's Momentary Time. Han Fei could deal with the Instant Mystic Spear, but not the Momentary Time, which could manipulate time in a small range. It was impossible for him to catch up.