Technical Support is handled well there, and combat errors are very few (roughly as many as Tolkien has)
A new take on Middle-earth - yes, that's exactly why many people bash it.
The sequel to "Henna's Adamant" turned out to be an absolute failure.
The style is a bit too epic and you get tired of reading by the end.
Overall, these are very different books - Tolkien has a sad-philosophical saga, while Perumov has gritty combat with some philosophical ideas.
I also liked The Black Book of Arda, but if there Niënna carries it with beautiful language and logical justification for the new perspective, then Perumov carries it through the combat, vividness, and epic nature of the narrative.