No glaring flaws or absolute turn-offs, but there is somewhat a lack of realism. Not in the sense that the story should be realistic, but in the sense that the story feels like a floating dream rather than an alternate world. The plot, settings, and characters all feel very separated with little connection to each other, creating a feeling of three separate layers haphazardly stacked on top of each other.
Other than that, it's an alright story for people who are interested in a historical war + game novel.
The feel of tension is completely f*cked, for a lack of a better description. The author consistently fails to set up reasonable difficulties, either setting them too high or too low, and then relies on a time-skip to fix it. All upgrade values are arbitrary and it's pretty obvious; MC gets his upgrade when the author decides its convenient.
There are also other issues, but at least the story is coherent, so 2.5.