FanficRecs Meh.
This fic's quality or lack of it largely depends on how much you tolerate OO Cnes and wide breaks from Dx D canon because the author changes a LOT. Issei loses any plot role or relevance and the Boosted Gear while having no bearing on the story, which is a turnoff to fans of the character. A OC based on Karasuba from Sekirei gets Ddraig instead and despite being Issei's twin has no real interaction with him. Likewise Shirou ends up as a definite case of spotlight stealing crossover character. I generally enjoy Shirou elsewhere, but here he's painfully bland and uninteresting. Rias and Sirzechs are made more unpleasant as well. While the writing quality is generally good, especially with the fight scenes, the plot is overly exposition reliant. There is good characterization for some individuals but for other it falls flat especially where it wasn't needed namely Asia. The author shows a lot of favoritism towards Nasuverse elements but lacks the skill to make them work. The tone is inconsistent and tries too hard to refit the Dx D universe with plot elements that don't even make sense half the time. This results in a mess of mood whiplash and elements at odds. It also wastes a lot of potential in turning the story into a generic insert rewrite that makes a crossover of two fascinating universes uninteresting. It's a mixed bag. Some aspects range from okay to good, but I would not recommend it if your a fan of High School Dx D or Fate/stay night. It's a disservice to both.
FanficRecs Review
Good story, it's a complete AU so you have to forget anything that is not basic in DXD but it's used very well.
A lot of World building well explained. Good characterization. Some characters a more likable. There are logical explanation like the differences with shirou is because he as 2 origin instead of one.
A must read
FanficRecs Avoid at all costs
A Demon's Lord Hero looks like it's a diamond, however, truth is, that sadly it is not.
Fahad has refused to read beyond the initial volumes of Dx D and insists on that he is doing things better... by utterly altering the Dx D setting until it does not resembles _anything_ like we know. Devils are a lot more selfish, Kokabiel is a bigger idiot and so on.
Issei is stripped of his most useful tool: The Boosted Gear and his accomplishments with it are handwaved away and given a swift kick in the ass. Issei won't become the Oppai Dragon, won't break the cycle between Ddraig and Albion, etc.
Why? Because the author has decided to give the boosted gear to the totally great Original Character: Karasuba, a bloodthristy psycho, who for all intents hates her brother and seeks to constantly pick a fight with Shirou.
Serafall's character is twisted so she is a sinful girl... as in her personal indulgence needed by all devils here is the Sin of Wrath. For reasons never explained devils now need to sin a lot, or they uh, idk. Fahad never specified the downside of it.
Sizerchs is bumped to magnificent bastard who works 'For the Greater Good.'
In short, avoid at all costs.
FanficRecs Definitely worth a read
I blew through the story over a long weekend, and loved every minute of it. I particularly like that the author isn't afraid to show the devils as more than 'people with wings', but as something a little Eldritch, powerful, and definitely Not Human. The writing is also well done, lots good descriptive language and some of the dialogue is laugh out loud funny.
That said, it's certainly true that Issei has zero plot relevance. For me this is a case of nothing of value being lost, but fair warning. The other common gripe, Karasuba, is something i wish more authors did. For Issei in canon the Boosted Gear is a wonderful thing that made all his wishes come true. For Karasuba, its as much a curse as it was a blessing, and causes her a fair amount of grief. This gets reflected in her character and in her relationship with Issei, and her character is better for it.