The first thing to notice about Impureza is that, especially on this album, they are a band steeped in Spanish culture and history. The lyrics are in Spanish, the touches come from Spanish folk music, all that. And yet, the band is based in France. Of course being based in a neighboring country isn't the other side of the world, and, more importantly, several members, including the guitarist handling the flamenco parts and the vocalist, are of Spanish descent does make the situation less of a headscratcher. I haven't felt any touches of French folk music or any similar influence, so someone listening to any Impureza without the knowledge of the band's base of operations would not be able to easily guess it.
When dealing with genre mixes where one side is metal and one side is not metal, there's always the chance that it would feel too gimmicky. Of course feeling at least a bit gimmicky is unavoidable but there's nothing inherently wrong with having a gimmick and having that gimmick be a genre blend, it's more of a question of whether one or both sides of the genre blend feel authentic or if they feel like a stereotyped version specifically for the purpose of the gimmick. Here's where my shortcomings come into play. I only have a very entry level knowledge of flamenco, so I couldn't tell you the difference between a "Hollywood story set in Mexico" flamenco and actual genuine flamenco the way someone who actually engages with the genre more profoundly could. I do know metal though.
So while I leave the "is the flamenco here authentic?" question to someone more prepared to answer it, what I do want to answer is "how well are the non-metal elements integrated and do they enhance the metal side?". The biggest gripe with such genre mixes are situations where the two genres are alternated rather than mixed. And there are moments like that on Alcázares, where acoustic flamenco would explode into technical death metal without any touches, and there's a significant chunk of the album that is purely technical death metal, but I'm glad that there's also a significant chunk that pushes both genres into each other.
What makes this more interesting of a mix is not only the alternation/blending ratio, but also having the flamenco side be more than just its most recognizable acoustic guitar signs, but also its percussion and horns being elements of the flamenco side. The blend works even in the most pummeling of moments, but I find that the moments where this mix makes the most sense are in the moments specific to prog/tech death where that very specific dissonant bass sound takes center stage in front of the guitars, leaving that space left open by the metal guitars to be occupied by flamenco elements. More so than on previous albums, I also find some of the elements, especially on the vocal side, to also have an "oriental" leaning, most evident on "Reconquistar Al-Ándalus", which does make sense given the Andalusian setting of the album.
As a person that has visited Spain and its Andalusian region for the first time this year, I recommend you not to go to Sevilla in the middle of the summer when it's 40°C out. As a reviewer, I recommend giving this serving of flamenco death metal a try.
