I think one of the main issues with this plot (aside from just being really shallow and dumb in general) is that it refuses to get attached to any of the characters. Most characters are introduced and then almost immediately discarded within a few chapters. The most prominent example is the MC's original party, who were killed off with little to no fanfare like 3-4 chapters after they betrayed him. The MC casually mentioned wanting to talk to the leader, but that just never happened. They all just kinda died and went away quietly, and then the plot forgot about them completely. Apparently being an S-ranked adventurer means very, very little in this world, since they stood absolutely no chance at all. It's funny that the artist discusses the design of the outfit of the leader in an omake when that character has pretty much zero relevance to the story.
But it's not just them, the elves, the dragonkin, the other demon lord and his meticulously designed subordinates, they die almost immediately and are never heard from again (special mention to the female subordinate of the other demon lord, who was apparently passionately loyal to him and whom he killed off with no regard whatsoever. It's like, maybe that would've been impactful if we knew anything at all about those characters, but we don't, so it's totally meaningless.) You can't really say there are any antagonists in this story, because everyone who opposes the MC and his friends are instantly killed off and forgotten. The king of Resaga is the only vaguely recurring character with opposing objectives, but he's barely a character at all. He has no name, no distinguishing characteristics and he only shows up for a single page a few times.
And, although they do have names and something like distinct personalities, the members of the demon lord army are also rather vapid. Of course, they are totally one-dimensional, but they're also kind of inconsistent. Like, here are Aria is shown to treat Licht unfavorably, but it hasn't really been established that she favors her other subordinates over him, so it kinda comes out of nowhere. All of a sudden it's implied that Dorothy has feelings for Licht, but that's also totally out of left field, with no prior precedent or even anything that would explain why.
Last on my list of complaints, characters are psychic. There are a few examples of this. Like, when they find the dead sniper in the other demon lord's castle, they mention that they found her dead already, so they didn't witness her death. However, the big fluffy guy then says she was killed using her own skill (she just has piercing wounds, how does he know that?). The other female character suggests maybe the enemy has a similar skill, but he rejects that explanation, saying she was killed in the -exact- same way as her own attack. Of course, this is true, but if he didn't witness her death, or the attack she performed prior to that, how could he possibly know?
Another example is the adventurer coming to persuade Berne to provide reinforcements to their dungeon expedition. He's like "there's a lot of magic energy in the dungeon, and we've lost contact with our initial investigation team, we have to go in there with force OR THE MONSTERS WILL COME OUT AND PROBABLY CONQUER THE ENTIRE WORLD". Like, bruh, what? Why would you draw that conclusion? I mean, yeah, it's probably dangerous if a squad of S-ranked adventurers went MIA, but given that you know absolutely nothing about the cause, isn't that kind of a huge leap?
I mean, the reason for all of this is pretty obvious. The author just isn't a very good writer. They forget which characters are supposed to know what, or can't be arsed to come up with a logical justification for why the plot ends up where it does. But it's like, really irritating when the dots don't connect.
Man this comment ended up being my ranty review of the whole manga, sorry for the wall of text.