Javier Falcon is a cool Spanish Chief Inspector based in Seville Spain. He is the brainchild of a British crime writer, Robert Wilson, who combined crime solving and the psychological development of the hero in a series of four mysteries. I think that understanding the author's background and intent may help readers understand some of the negative reviews of this highly entertaining and sophisticated work.
One reviewer complains that Falcon makes mistakes. In my view this is not a weakness of the work. Wilson's characters are fallible and thus human, credible and interesting.
Other reviewers complain that the actors do not affect Spanish accents. Well, it is an English language production. The Spanish setting for the story is well evoked through the scenery, the backgrounds of the characters, the sense of heat, a bull fight and the legal system in play. I was not at all distracted by the actors' natural English accents. If anything, they contributed positively and reinforced understanding of roles.
I am inclined to disagree with the reviewer who found the gore was extraneous. Shocking, yes, but it was relevant.
I agree that the sex did not have adequate lead-up. I too found the hero's constant drinking of bottled water tiresome. I have no idea what powdered street drug he uses to spike it. It certainly does not seem to alter his behaviour.
Yes, the plot is complex. Another strength. You will likely be left with questions and possibly a wish to watch the film again for the answers. It is that good.