I think it stems from a Park's not having known where this story was going when he started it. That's my perspective as an outsider, but the story's drastic shifts and huge, convoluted cast of characters doesn't seem like it was planned out and--well, wait, let's back up.
RAIDERS follows Irel Clark, assistant to famous archaeologist Wilter Langhem, who specializes in Biblical artifacts. Guessed where that's going? Yep, he's searching for the Holy Grail. And right in the first chapter of the comic, they find one of the mythical artifacts of Christ - one of the five chrism bottles in which his disciples collected his blood as he hung on the cross. Of course, they had to raid (aha, the title that makes no sense concerning anything else in the series [it really doesn't overall] does at least have an inspiration from the book) Scotland's Glastonbury Cathedral to get it and accidentally destroy it during their escape.
Well, Langhem isn't the only one looking for artifacts related to Jesus Christ and his and Irel's battle with one such group, and later an inhuman monster by the name of Lamia--both Lamia and this paramilitary group seek the artifacts for their own purposes--lead him to believe they may have found a genuine artifact for once. This sets off a chain of events culminating in a lot of death and Irel drinking the blood of Christ, miraculously still viscous after two millennium, hoping the myth of eternal life is real and can heal him of his otherwise-fatal wounds. It is, it does and it opens up an even bigger can of worms.
Basically, we learn over the course of the series, everything from the Bible is false, except for some of the stuff about Jesus and his miracles. Jesus himself, along with Judas and Peter, wrote the Bible as fiction (Judas claims he openly stole from the myths of various religions around the world and he can't believe nobody's ever called him on it) to lay the foundations for, essentially, world conquest. See Jesus was not only real, he never died. And he thinks of himself as God, because he's some sort of human mutation who is an organic incarnation of the energy from the Big Bang and, well, he had some time on his hands his first few decades of life-in-hiding, so he and his pals wrote that Bible dealy.
Along the way, to further their goal of eventually eliminating humanity (so Earth can be the peaceful Garden of Eden it was before people mucked it all up and Jesus can quietly rule the Earth as he feels he was intended to do from the start), they have created an army of "zombies" - humans restored to life and imbued with special powers (and with their memories and personalities intact for the most part) via elixirs distilled from Jesus's blood. And what do you think Irel's drinking directly of Jesus's blood will bring him? Yeaaahhh....
Okay, so... this book is certainly action-packed. Like Michael Bay movie action-packed. If you're into that, cool, cool... this is up your alley. I don't dislike it, but there were entire volumes that were essentially a single battle. It gets old, you know?
More than that, as I said, Park didn't seem to know where the story was going when he started. Like, not a clue. It begins as an action-fantasy revolving around archaeologists and devolves into a superhero-esque smash-em-up and stays there. Many of the big questions that it initially asks about the relationship between faith and reality (it posits, through the mouth of Jesus himself, that reality has no impact on religion because faith is independent of it; that's either a good description of organized religion or a damning one depending on your perspective) and the strong's obligation to the weak (again, from Jesus's mouth; he is strong, he has an obligation to the weak to protect them. In this case, humanity is its own enemy, so it's gotta go.) are left by the wayside. Along the way, a multitude of characters are added, really so many that it's hard to keep track of them. Many have their own subplots going concurrently and Park shifts haphazardly between them - a few pages here, a few pages there--within chapters--sometimes, it seems, simply to fill pages. This gives the middle to later volumes of the book a schizophrenic feel that I didn't like at all. It really broke up the cohesiveness of the story to the point where I stopped caring about certain parts a few times. The storyline at Area 51, for example - I just wanted them to get it done and move on.
Character development is spotty, at best. Irel starts off as one of the more well-rounded characters, but as his powers grow, his character becomes more two-dimensional. On the opposite end of the spectrum, Lamia, who is originally presented as a terrifying monster, drawn in a fairly-angular style and made to look rather ghastly, becomes more and more human--and Park draws her in an increasingly softer, more human and much more feminine style, as her personality develops. Most other characters remain static, except for the ones who become "zombies" and gain powers--which hardly counts as character development, really--and Grace, Langhem's daughter who is initially presented as a tough-cookie, smart-and-pretty combo kind of character, but is quickly relegated to comic relief. I didn't understand that one; didn't make a lot of sense.
The art is also uneven. The main characters are drawn in a manga-influenced style that I'd give a 6 out of 10 if I had to rate it, but as the story mostly takes place in Europe and the US, and to differentiate the plethora of secondary and ancillary characters, the majority of non-main characters are drawn in a western comics style that, frankly, Park sucks at. It's not just not as good as his manga style, it's objectively bad. Perspective and anatomy issues abound and characters are frequently off-model. Park's line art is razor thin to begin with and when he uses the manga-influenced style, he can make it work (usually) through the use of negative space and toning as appropriate, but in a style he's obviously not as comfortable or proficient with, that weakness of the lines serves to highlight each and every shortcoming. I will say, he gets a little better at both his main style and the western-influenced style by the end of the series, but it's still not great art. I'm a little surprised Daiwon C.I. published it as is.
At bottom, I'm not sorry I read this series as there was enough good about it that overall I enjoyed it. But would I recommend you run out and buy it? No. If, however, you see the first volume cheap somewhere, as I did, you might give it a chance and see what you make of it.