This is a weird situation that I haven't seen often. The manga and anime, both, significantly improve on the light novels. The reason is because the author doesn't seem to understand the concept of a scene. It's not surprising to see amateur writing mistakes in Light Novels, but this one in particular makes a few egregious, unnecessary errors.
First, I'll start with the positives. Despite its flaws, the story, world-building, and characters are very charming. It's easy to get sucked in because the author did a fantastic job of setting up good mystery elements and maintaining the effort to feed them along the way. I'm completely enamored by the premise. It's fairly generic but unlike most other attempts in the same (or similar) genre, it takes its time to develop the mundane. Doing so, greatly pays off the more you read. While the MC isn't the most interesting of characters, he carries the story adequately. I definitely got the full sense of what it means to be a dungeon-diving adventurer. The anime and manga both do a great job of relaying this too. I've seen quite a bit of criticism leveled at this work for being "too slow" or "boring." While not what I would consider entirely slice-of-life, I would argue that works in its favor as part of the charm.
The negatives, for the light novel, are more complicated and have to do with, what I consider to be, poor technical writing. The first issue is the absence of scene. Everything is very linear. Character A goes to house, leaves house, walks down the road, arrives at Place, enters place, talks to Character B, exits Place, travels to new Place.
This is the primary reason why I consider the Manga/Anime to be superior to the Light Novels. In the Manga/Anime, scenes are properly constructed. This obviously means they take liberties with the source material and there is quite a bit of re-arranging of events and dialogue. With that said, it greatly enhances the experience. Now, the linearity is not the full story. If it were, you could raise issue with my criticism. However, while it goes from point A to point B, it meanders heavily with an excess of asides and internal monologuing. Sometimes it works and you get a better impression of what has happened, is happening, or will happen. Most of the time, imo, it amounts to inconsequential flavor text. This is where it comes off as a soapbox for the author to describe and diatribe about anything and everything. I think it shows a good imagination on the part of the author and a good attention to detail, but this is why fictional literature relies on scenes. The author drags us through far too many of these asides which hamper the feeling of progression. It lulled me into such a pattern that when something, relatively, epic was happening, I was reading the same as when the characters were strolling down the road. It lost its impact. Which is very unfortunate because I do think the major plot points are very interesting, but getting to them is not, for the most part.
I don't want to delve into spoilers too much, but [vague, minor potential spoiler ahead]
there is a certain plot point which I felt devalued much of the premise of these books. They gain a sort of unnatural over-powered McGuffin (let's call it) that felt totally unnecessary. This was supposed to be a story about a guy ri slowly and steadily to ever greater heights. With that said, ofc, there's not much use for it nor do the characters even consider using it. It's obviously for much later in the story but I was a little sad at how ridiculous the implications were. Especially in a series where so much attention to detail was extolled.
There's other technical issues, but most of them are fairly widespread throughout the LN industry, it's not enough to take off points or bring attention to them here for the sake of nitpicking. First person narrative is a very difficult perspective to write in and almost no one does it well. I would highly recommend the manga and anime, but while I thoroughly enjoy the Light Novels, I would hesitate to recommend them unless you're heavily invested in either the manga or anime.
