Story
Not a bad story overall but lacking in execution and a bit jarring with how time flows between events. The setup of the plot in the beginning is good even though it differs somewhat from the original source but ends up failing a bit with how the characters behave and how the entire conflict and resolution unfolds, it's very lacking. And the same pattern forms for the rest of the anime where it begins well with the setup but ends up with a very forgettable plot where things are feel like it's happening for the sake of following a script, not a story that's unfolding naturally. Looking back at the entire thing, it doesn't feel like it's a bad story just that it could've been better if it was executed with more care I think.

Characters
I already mentioned a bit about the characters in the story portion of the review which is character behaviour being a bit lacking. What I mean with character behaviour being lacking is their reaction to events and certain scenarios. I don't expect perfection, of course, but there would be times where I would fine myself wanting more of a reaction by the character due the circumstances because it would be more natural but instead they are either too stiff and overly done or made to be too flat, MC being a main example. On a more opinionated comment towards the characters, it felt very cliche and overly done trope, especially for the main female characters. It's based on preference and how everyone perceives it is different but I find it as a detriment to the story when a certain trope is being done that sacrifices character likability and world building, namely the last portion of the anime.

Setting and World Building
This is also another area where the anime seems to struggle but has some good points to mention. The places where the MC visits and gets involved with are nice to see and interesting, a lot of it shown through the art and design which is great to see. However aside from the design, the world doesn't feel contiguous, as in it doesn't feel like it's set in the same world or plane of existence. The MC travels from place to place but we only see glances of the travel terrains which is great but there's no sight of civilization anywhere in those places. The cultures and appearances are well done, it is convincing that it's a different place just by seeing the architecture and clothing of the people in the land. The kingdom where MC starts in is not well done though, it is not convincing that it is a powerful kingdom and it's very difficult to understand how it's so prosperous witnessing all the events that happen around the MC. A princess of the kingdom is kidnapped right under their noses and their nobles run rampant in the capital doing whatever they want without repercussion, not even inside the royal castle are they put under watch even as hired soldiers in the army. To give an example, like Charles being able to execute torture and interrogate the MC even when direct soldiers are dispatched to watch over the kid and not treat the kid poorly, that all happens inside the castle and

Art, Design and Animation
The art is is great, it's exceptional and very pleasing to see along with the designs of the settings and the characters! The animation is only okay, it's not something I would write home about but it's not so bad that I would stop watching it, the same could be said about the fight scenes. I think the art and character designs were the more outstanding points of the anime, it's quite above average with how beautiful, stunning the characters were, yes male characters too, and how attractive and cute they appear.

Sound and Music
This was also an unexpectedly well done part for the anime, it was kind of awkward in the beginning due to how epic the start of the ending song is but the music choices were great, I loved the OP and ED. As for the sound effects and BGM during the anime, they were not disturbing and sounded fine alongside what was happening on the screen.

Conclusion
I think this anime was done fine, it's not something that I would recommend everyone and anyone but if asked for an isekai harem with an OP MC then I would at least mention this one. I found myself just watching the entire anime and not feeling like I want to stop, just up to the last 2 episodes at least where I found myself more annoyed than uninterested with the plot. So if you are looking for something to watch and you like or don't mind cliche stories with cliche tropes then Seirei Gensouki is a fine watch.