After the very good first arc the pace increases until everything feels weird. Like you are reading the spark notes of a stroy. More and more of the story involves bizzare coincedences and weird decisions, especially in the last arc.
Some annoyances and major spoilers :
In the second major arc, it is weird the grandfather left them to basically die instead of just killing two people that are quite clearly evil, which would have taken maybe 5 minutes. He could at least hide for a month to see what happens when he "left", it would have been enough. He messed in just this way before and shows zero growth. The fact Woo-Moon did somehow win in the end is only due to plot armor. It was very unlikely and hence insane action on the part of the Grandfather. The fact the mother was completely useless in the story is a bit sad.
The last arc is even more contrived. First of all, the younger disciple is pulled in completely by chance. Then this is discovered weeks after as she leaves just a day before Woo-Moon returns. Then even though she zombie walks for several days he somehow doesn't catch up only to arrive at just the right time to the right place, which also happens to the grandfather (let's say at least he was looking for Mucheon whatever guys) and the spear guy who appeared completely at random.
Then they fight, and Woo-Moon randomly goes a very convenient coma which makes him separate from Mara and become five times more powerful. That is just bad writing.
And these kind of random encounters continue to be the backbone of the story. They don't just happen, this is all that happens. When Woo-Moon was just about to start his complicated plan for fighting the Sand guys, he enconuters an insanely powerful person who is already hell bent on destroying those same people and both of them would have likely been enough, but instead of telling him his planm, he lets him leave without attempting anything when the other has a PTSD episode and stumbles away due to the mention of the word "comrade" O_o.