It's a series that had potential, but ruined it by doing too much.
[mild spoilers pertaining to storyline progression and general point-of-view]
A third season of Spice and Wolf is perhaps the most desired adaptation among fans of the franchise. When announcement came through that a similar series, with similar staff members, was going to be released, fans were ecstatic. Unfortunately, it's not quite that simple to live up to the name of the predecessor, and it's even more difficult to satiate an entirely new crowd. Maoyū Maō Yūsha suffers from a variety of debilitating features that not only make it a shitty, half-assed 'successor', but an awfully trite,mundane standalone.
Maoyū Maō Yūsha doesn't have the world's most amazing storyline. In fact, it's relatively archetypal -- the king of demons and the hero of humanity wish to team up to ultimately better the world. Sounds relatively creative right? It's not. But it could have been. We're given two main characters, Maou, and Yuusha [translated to Devil and Hero]. Their names are titular of their roles. During each episode, humanity attains a new invention that may better life [e.g. new farming techniques, agriculture]; and during each episode, a mild time skip is applied [typically a few months]. What does this do? It advances the plot. But does it do anything more than that? Probably not. Is it bad because it's simple? Not necessarily. Is it bad if they advance the plot over developing the characters/mood/every other aspect of the show? Probably.
A certain series revolving around a travelling merchant and a wolf goddess revolved around the characters -- every episode moved at a snail's pace, and the characters were developed accordingly; in Maoyū Maō Yūsha, it isn't uncommon for several months to move in one episode. The characters aren't developed. There's a romance between Maou and Yuusha. But the romance between the two are magnetic. They're only together for the storyline. A magnet doesn't need a reason to attract the opposite charge, neither does Maou to Yuusha, and Yuusha to Maou. They're together, but there's no semblance or inclination that would insinuate anything outside of being together for the storyline. Love? What's that. 12 episodes, or several years, resulted in a half-assed display of affection at the end. No discourse implying anything significant, and no actions outside the cursory depicted a sort of love outside of obligation.
The art and animation in Maoyū Maō Yūsha was fantastic? You mean the art and animation during Maoyū Maō Yūsha's first episode was fantastic. Arms was tasked with animating the series -- Arms did a really nice job during the former half of the series. The latter half, they kinda just gave up on maintaining consistent quality. Surprisingly, Arms, a studio, known for their integration of ecchi into everything, didn't put that much ecchi into Maoyū Maō Yūsha. That's a bright side. How about the soundtrack? The soundtrack's probably one of the best, redeeming features of the series. The track titled 'Beyond the Hill' by Takeshi Hama [composer of the original Devil May Cry soundtrack]would on its own, bump the score up by at least a point. The series contained a fantastic soundtrack, albeit, it wasn't exactly applied that well. The general soundtrack is perhaps one of the few things that Maoyū Maō Yūsha does better than its related series. It may not have been used as well [the soundtrack in the former was heavily ambient, but that ambience was integral in developing the mood], but it's certainly noteworthy [the main theme really is good].
So, we're given a series with generic characters that are seldom developed, the noble goal to save the world, time skips for plot, alongside a fantastic soundtrack, inconsistent animation, and a predictable, archetypal plot progression. Oh, and don't forget the romance. Everybody loves romance. So to sum it up, we're given a series that attempts to do too much within 12 episodes, that's understandable right? Well, it probably would be if it attempted to do something difficult. Every other development and concept explored in Maoyū Maō Yūsha isn't exactly novel, or avant-garde; it's not even a rendition of generic concepts. They're really just generic concepts without any sort of application. Economics? Spice and Wolf had more economics within a single episode than this did in 12. The notion that opposite sides often have a lot in common and that they probably shouldn't kill each other? That's clearly edgy and new.
It's not as enjoyable as it could be to watch because the series tried to do too much. When you introduce a romance, actually develop it. When you attempt to mirror a similar series, at least compliment it. The series doesn't revolve around combat and the mechanics of the world? Then don't dedicate an entire episode to combat. There wasn't enough time to do what we wanted? Then don't do so much.