So the general premise is something we have seen before, where you have an OP main character who gets inserted smack dab in the middle of an unfamiliar universe. And you have the usual cliches of people underestimating them and not knowing their true power etc. And, this main character at first glance seems extremely laid back and chill, and does not like to intervene in things unless necessary. The plot can be slow at times, and some of the side characters also come off as annoying at the beginning, so I can understand how this has made some readers frustrated. You end up situations where the MC could easily resolve whatever conflict or misunderstanding is happening but chooses not to. So what is it that makes this manhwa stand out?
1. The setting
While the worldbuilding isn't super comprehensive, the setting of sci-fi space martial arts is fairly unique + the art is pretty good. I especially liked how the triad planet backgrounds were drawn.
2. The dialogue
This is the main one, and what most surprised me reading this manhwa. The author isn't Shakespeare, and it isn't like the manhwa is full of literary devices masking themes about life and philosophy that your high school english teacher would make you write essays about while you think to yourself how its all bullshit. But some of the dialogue actually does say things that are somewhat insightful; not just about how the characters think and why they act how they do, but how we think and why we act in certain ways as well. There are moments where it does fall short, but most of the time the main character actually does feel like someone who has lived for 2000 years (although it is important to remember that he was solitary the whole time, so it isn't like he is as wise as someone who would have interacted with society for 2000 years). Power/strength, sacrifice, the value of life, and purpose/goals are some examples of themes that appear throughout the manhwa, with varying degrees of subtlety. Like there are some real bangers in there.
3. It is funny as shit
Maybe some might find the comedy to be out of place, since it is frequently the kind of dry humor that unexpectedly appears right after serious subject matter, but personally I find it to be funny as hell. Although I do wish the author didn't reduce some of the characters (like the tang woman) to comedic relief as I'm pretty sure she became stupider every arc for some reason.
Now for the complaints.
Every manga/manhwa in this genre will have some stupid characters, cliche braindead antagonists that try to fight the MC, etc. The issue is that some of these characters stand out even more here. Some illogical side character behavior looks even more head scratching when you compare it to the MCs behavior. Some examples would be the current heavenly demon (although you can argue he is young and easily manipulated by the demon soul), and the heavenly demon sect grand elder who acts kind of strange early but more logical later.
The second main complaint I have is character development specifically in the MC. He goes through some random mental enlightenment and gets stronger randomly like twice, but I wish we could see more emotional development in him. Even after like 80 chapters he randomly says some bullshit about how he doesn't mind throwing his life away for someone. This felt like it undermined what the author had set up in the last 80 chapters, where it seemed that the MC was finally learning how to live for himself after basically being used as a mindless killing machine and then choosing to lock himself up for two millennia. But hopefully the author does develop his character more moving forward.
Edit: I also forgot to say kudos to the translator team, they definitely did a great job translating the dialogue