Most of the faults of this honestly fall on the staff of the manga, rather than the original book author.
The art is generic and boring, neither looking pretty nor unique. The antagonists all have similar character designs, to the point where you can barely tell who is who; this is especially bad in later arcs. The action is AWFUL; the artist clearly can't draw dynamic poses, resulting in almost every action being between two guys who look like they are casually standing around with a sword as colorful lights scatter around them that are supposedly sword slashes.
The writing is, as the other person commented, incoherent very often. You'll go from one chapter to the next and wonder if you skipped something... but nope, the manga writers skipped something from the novel. Hell, sometimes they skip things like massive fights! It's really awful and hurts the story terribly.
The pacing, though, is the worst part. Early on, this was honestly fine, but by the later chapters, they were rushing it so badly it felt like literally nothing had time to breathe. You'd have villains introduced, fought, and killed in 4ish chapters, which given the chapters are 10 pages, is like 40 pages for a supposedly major villain. Over and over. Often times I'd be like "Oh wait, that guy was the one responsible for that?" because I didn't have time to figure out who they were. No time for romance, no time for intrigue, no time for character development. Hell, even the power-ups got rushed through, with him often just pulling them out of thin air.
And if all that wasn't bad enough, they didn't bother to finish. They just suddenly cut it off midway, with major plot elements unresolved. The main character supposedly had this major villain who wanted to take over his body... and they just never address that. One heroine was in need of resurrection, and they mention a person who could help with that in a foreign land... and it just goes unresolved. Hell, the main character made a promise to marry one girl, and then when he finally gets her back from the people who captures her the story ends before he can.
This is one of the worst adaptations I've ever seen. I feel sorry for the author. I can see where they butchered his work. There were definitely flaws, but they were more generic, like "too many villains" or "repetitive plot arcs", not these awful problems the adaptation has.
The translation was also not great. Early on it was fine, but the later translators all kinda sucked for various reasons. One translator for like 20 chapters was unreadably bad english, but even the main one who covered the tail end had a severe problem with character names, choosing stupidly to translate them literally, rather than just use chinese names, resulting in names like "Cloud" and "Pretty" instead of "Xing Yun" and "Xia".
Overall, can't recommend.