@JustNatsuki
I assumed you were after face slapping, as you stated, "... I came here for power fantasy trash"
Now, your question is confusing to me.
Firstly, he's already using some of his power; hasn't he already beat a bunch, killed another and is currently beating Jia Yunfeng?
Secondly, Lord San Shen and the master of Night Fortress, whom Ming Feizhen happens to be, weren't fighters of justice; they were associated with evil. The only "pure" people introduced thus far in the manhua are Su Xiao and Li Hongzhuang. Ming Feizhen isn't an enforcer of justice; he's a man trying to fix past mistakes, protect friends and move on with his life.
Thirdly, right from the beginning, it was established that not even being invincible, as Ming Feizhen was, could fix all the problems. That led to him choosing to stop involving himself with the pugilistic world. In other words, revealing his powers didn't fix anything. Instead, he left a mess he's trying to clean up as we're witnessing. In short, might doesn't solve all of the problems in this world and evidently created more.
Fourth, revealing his various identities would jeopardise all those around him, so it hardly makes sense for him to use his unique skills that'd instantly reveal his identity. For the record, he doesn't have many unique skills. He's trying to lay low. Also, MFZ has yet to run into anyone who's challenging enough to force him to go all-out.
In summary, if you're here to see him physically slay evil at every turn, you're in the wrong place. If you're looking for a story where good and evil are blurred, and not every problem can be resolved using violence, welcome aboard. Hopefully, that's a sufficiently comprehensive answer.