This manhwa had great potential. When I first started reading I was very excited to see where it went. The characters were funny, interesting, and mysterious. The fight scenes were passable and fresh enough to interest me. The villains managed to be intimidating even though the main character could destroy them without a second thought.
The main characters are still funny and interesting, but scratch everything else.
Every arc is practically the same (Saying practically is being very generous). I'll lay out how almost every arc after the first few go.
1: Bad guys come for nefarious reasons.
2: They kidnap people, most often the useless kids that hang around Rai.
3: M-21 and Friends go to rescue them.
4: They are beaten after giving it their all and lamenting how weak they are.
5: Stronger person comes to save everyone. Most often Frankenstein, but sometimes others.
6: They do well but still can't beat them.
7: Rai shows up and it's over.
There you go. If you aren't interested in the character interactions or humor that go on during and between arcs then this manhwa has nothing to offer you.
The villains are extremely boring as there are almost no consequences to anything they do. Most of them have the same copy and paste personality. As the series goes on the designs get increasingly stupid, culminating in a blueberry colored Majin Buu werewolf thing that made me exclaim "Wow this is dumb". Here's a few lines you'll hear from just about every villain in the series.
"Haha you think you could possibly defeat me!?"
"Hey you're not bad!" (There's seriously one guy that said this like 10 times in the same fight unironically)
"How could I be losing against someone like you!!!1!"
Then they die.
This may seem harsh but it's annoying to read something and know exactly what will happen next without fail.
Give this manhwa a try, you'll probably really like the first few arcs like I did, but try not to expect much more than that.