Well, if you want a fantasy adventure with dark tones and likeable characters, this is a must read. The first 10 volumes are the main story, the last volume (11) is 1/2 epilogue and 1/2 side-stories, so it pretty nicely wrapped up. The plot has twists, the dialogues are really engaging, it has fked up scenes (dark/gory stuff), funny scenes, the magic/sorcery system was interesting, action and a decent world building. The art was nice, and it had a good amount of illustrations.
The anime covers only the first volume, I still would recommend reading at least the last chapter of the first volume. You may even skip it, I don't think that is a deal breaker.
I think that the most crucial part of this story is liking the characters, I did. The main duo is one of the best I have ever seen and I loved the dialogues between them and how they slowly changed with every chapter. There is a lot of development in general, not just them. It makes you even care for the side characters.
Romance is not the focus but it's present and there is progress.
One difference between LN and anime is that for the most part, the story is told from the point of view of the mercenary and what he is thinking. This gave a different feeling than the anime.