Listen, i enjoy shonen more than the avarage reader, but saying that this manga it's generic it's not enough to put into perspective how formulaic everything is, if you have read a decent amount of shonen you're going to be able to predict every plot twist on the series. Also words can't describe how forced some of the "sentimental" scenes are in here, for the most part the majority of secondary characters are introduced on an episode and you're expected to get sentimental over their situation even tho they had barely any screen time. The humor it's meh. The characters designs are ugly as sin, specially kid Jio who is the definition of overly designed. The idea of hunting rare O parts sounds fun, but that gets immediately forgotten in order to have more fights and that wouldn't be a problem if the fights where good, but they're not, the fights are extremely repetitive, formulaic and predictable and if you hate mangas like Fairy tail where the power of friendship wins everything, BOI this manga it's definitely not for you
there's one scene and a characters who made me consider giving this a higher score, Kirin. He's the best character on the series and almost every piece of good writing it's dedicated to him and while I'm not a fan of the fights in this manga, the resolution of his fight with Kujaku was extremely satisfactory and by far the best fight in here, it's a shame that the chapter following that fight it's one of the worse episodes of the series.
Also yeah, the ending it's a complete mess with a lot of plot holes but that's whatever, I'm more disappointed over the fact that the undertones of life being nothing but a cycle of information never got the time it deserved, seriously that's the most interesting stuff in the manga but instead o talking about that they talk about friendship and the value of feels? You could at least tie both of those together and have an intellectual fight or whatever, but that would require original writing and we can't have that.
If i had to say something good about 666 Satan; the Demon/Angels designs are on point.