Rant warning Ok so after taking a break during S2 while waiting for chapters to build up, Ive returned and reread it from the start, so these are my thoughts after binging it. Art: Art is good, really solid and consistent all the way through. Characters: Given this is a shounen revenge type story it makes sense that most of the focus is on Unseong but it feels like the side cast gets neglected a ton. His two squadmates (the sword captain guy and the stealth leady) were amazing to see, and they each had their own reasons for respecting him. Seeing them grow with having Unseong as their inspirationwas great.
The two masters were great but didnt have a lot of screentime. The spear master... dies in chapter 1 and we see him in like 3 flashbacks? The demonic master, seeing him treat Unseong like his son was wholesome, but its also tragic how he dies almost as soon as he is introduced.
I'll be blunt. Ayeong fucking sucked. worst girl. To explain, she did nothing. all 4 seasons. SHE LOST ALL HER FIGHTS. AND THEN SHE GETS THIS MAGIC "DIVINE MAIDEN" POWERUP OUT OF NOWHERE AFTER SLACKING OFF FOR YEARS. Its even worse in retrospect because the divine maiden powers did had no plot relevance besides the end of season 2(For stalling the Jiangshi). Other than that one high-stakes moment, all she does is use them on fodder in the war. All she does is steal screentime with 0 payoff: Season 1- oh youre strong kid hehe ur my rival now! ooh oof I used 1 ability and now Im out of qi guess Ill forfeit, youre welcome Im going easy on you. Season 2- Help my squad got wiped and we need backup Season 3- Oh I guess Im the divine maiden, does that mean you'll marry me? Oh boohoo Im so weak please teach me how to get stronger. (Neither of these scenes payoff because Unseong essentailly goes "Ill get back to you" and "Put on these weights while I go on vacation") Also, the stealth girl should've been the love interest, during season 2, she had not only more screentime but more MUTUAL interactions with Unseong, compared to him seeing Ayeong as a waste of talent at that point.
She is the potential man meme. They call her 007- 0 feats, 0 relevance, 7 ass beatings. Give me liberty, Give me (divine) fire, Give me offscreen fodder or I retire. Master my own technique? ❌ Rely completely on magically granted divine maiden powers ?✅
Wait a minute, did she even finish mastering her original technique? The one from the beginning? I only recall her using the bell and the Divine fire in seasons 3 and 4, but thats the first heavenly demons power not her own fire is it?

Plot:
As Ouya said, seasons 1 and 2 were well paced, but seasons 3 and 4 are noticeably... worse. Season 1 makes sense as an intro season. Season 2, was a bit fast how quickly Unseong rose in power but its a shounen so eh. Season 3- The main antagonist changes, and a lot more characters get introduced. Season 4- OKAY THE BATTLES ARE LITERALLY ONE CHAPTER LONG NOW, WHERE IS UNSEONG?? I DONT CARE ABOUT THESE CHARACTERS WE HAVENT SEEN UNSEONG IN SEVERAL CHAPTERS. In season 4, the pacing goes into overdrive, you can tell by how battles are literally cut and skipped to the end result. THE FINAL BATTLE WAS THE MOST UNSATISFYING SHIT EVER.
Seasons 1 and 2 were satisfying in seeing Unseong's training and hard work pay off, but at the end of season 2, it felt like the author changed their mind in how the plot should go? Because the climatic battle as a deus ex machina- the heavenly demon who should be resting, comes to save the day and oneshots everyone and then dies. The Vice commander who had been built up as the big threat for 2 seasons, GETS ONE-SHOT AND UNSEONG DIDNT EVEN GET TO FIGHT HIM???
Season 3, my favourite part was when he went on "vacation" and journeyed for a while. Really helped expand the worldbuilding because he had been cooped up in the sect mostly or on high stakes missions. This arc felt like a break from everything. Him taking in a disciple was wholesome too. But after that, theres the whole Justice blood sect and the captial impostor emperor stuff thats all complicated and stuff. A lot of characters get shoved in and its hard to care for any of them.
Season 4- They lost the plot. It felt like they just wanted to end the series. Several chapters follow the justice blood sect members and not Unseong= filler. And then the final boss shows up, Unseong says they are beyond divine level and loses, but gets a powerup?? That body transformation technique from season 1 and reintroduced in season 3 as the only way to get rid of the poison in his body but would be infinitely harder to pull off at his divine level?? That originally knocked him out for days??? So he pulls that card, and it only takes a few minutes, and it also transcends him beyond divine level. Before, he threw all 1000 techniques at the boss, but that failed, so he realized he only needed one move. and its not the iconic heavenly dragon OR night spear technique. Its destruction wave. The author really gave up at the end. Its like they arent even trying. Its chapter 243. LOOK FOR YOURSELF HOW QUICKLY BARGIN-BIN KNOCKOFF MAHITO GOES FROM TALKING SHIT TO SHITTING HIS PANTS. They should be at the same level of power? or at least relatively??? If the final boss goes out without even a struggle, thats just unsatisfying. Season 1 fights were better than this shit.

Sorry about my rant. I did like this series, I have it an 8 for seasons 1 and 2, but seasons 3 and especially 4 have knocked it down to a 5 for me. I just finished binging it so the atrocious ending is still fresh. THEY EVEN SHOW BEST GIRL (EUNJI BAEK/ STEALTH LADY) BEING SAD OVER THE MARRIAGE ANNOUNCEMENT. ARE YOU KIDDING ME. Unseong isnt even in the last main-story chapter.
At least the author acknowledges the story and direction changes from season 3 onwards in his authors note.