the first "modern" thing the MC introduces is rubber, which he promptly uses to create condoms, like wtf?
first of all is the personality of the MC. He is supposed to have lived for several centuries before he experience the time rewind, but he acts like a teenager for the entirety of the series.
Next is the amount of plotholes in the series. For example, the main love interest's cultivation was chosen because it would grant her several lives when she's done cultivating, but then when the team cultivates further it is revealed that they gets reborn after they die with only a level loss as a penalty. This is just of countless plotholes and loose ends in the series.
The biggest indication that the author is clueless about where to go is that the big bad the MC lost to in the original time frame was only a shadow and the real thing is several times more powerful.