It's amazing. You'd assume it was written by an twelve-year-old. (I'm not just slagging it off, by the way. I can't think of anything closer to its content and tone than the kind of handwritten wank fantasy you'll find in the unused exercise books of an twelve-year-old boy.) It's the kind of show that's so bad that it'll attract reviewers who want to demolish it afterwards.
The key fact is that it's based on a web novel, i.e. internet fanfic, except that it got picked up for manga, anime and light novel adaptations. Its protagonist, Rein Shroud, is:
(a) a beast tamer (i.e. supposedly weak and useless), but he can do lots of things impossible for anyone else, e.g. control any number of animals simultaneously, or forming contracts with "ultimate species". No effort, danger or drama is required to gain these abilities. He has them from the beginning and he hadn't even realised that other people can't do them too.
(b) when he has to fight anyone, he can summon a swarm of poisonous birds or paralysing bees that knock out his opponent, no matter how strong. By amazing coincidence, these will just happen to have been flying around in the vicinity. So he's also invincible in combat, with no effort or danger.
(c) he then gains additional inherited powers (e.g. immunity to death, and I'm not joking) every time he recruits another "ultimate species" into his harem. Yes, they're all young, female and besotted with him. How did you guess? He doesn't do anything special to recruit them either. He just wanders around making the obvious choices and continually picking up superpowered girls.
(d) everything he does is greeted with amazement and disbelief at his superpowers, strength, inner beauty and/or impossible self-sacrificing altruism. No matter what it is, even if it's such a no-brainer that we'd have called him an idiot or a bastard if he hadn't done it. For instance, in ep.2 he accepts an arm-wrestling challenge from someone who looks stronger than him, with the prize at stake being his new friend Kanade. He only wins because of the magical strength he'd inherited from her, which he hadn't known about. Does Kanade bite his head off for making her the prize in a bet? Does she walk away and never come back. No, she says she's "happy that you protected me". Bloody hell.
Or, alternatively, in ep.11 he's trying to remove the slave collar of a little girl chained up in a dungeon by a sadistic woman-kidnapping bastard who's probably taking sexual advantage of his captives (although this is a family-friendly show, so there's no hint of that bit). The show wants to make him look heroic. "No one would be sad to lose me," says the little girl, trying to persuade him to abandon her and stop trying to help. Naturally, she'll be astounded when he keeps going. No, no, no, no, piss off, no. I'm pretty sure 10000000000000000 little girls would have all chewed their own legs off to escape from that dungeon, while abandoning her and departing was simply not an option for the hero of almost any narrative at all.
He's amazing. He has no character traits at all, unless you count "self-effacing". He's an empty space in the centre of the anime, taking the only permissible choice for 13 episodes and having everyone swoon in astonishment at how amazing he is. He makes the personality-free heroes of other harem/isekai wank anime look like Dostoevsky.
The "best" bit, though, is that the script has got the art it deserves. Rein doesn't even look human. He's too vacant and mindless for that. His facial expression looks as if he's got a key in his back that needs winding up.
Meanwhile, the baddies' dialogue might as well have been "I'm evil, bwahahahaha, look how evil I am, see me kick these puppies". In D&D terms, their alignment is Stupid Evil. So, for instance, the Hero's party boots out Rein at the start of the series, under the impression that there's no point in keeping around someone who handled their logistics, helped carry and transport things, controlled multiple animals simultaneously to perform reconnaissance and participate in combat, and then for luck helped out in combat himself too. And even if he hadn't been doing all that, five warm bodies are better than four. Ask any roleplayer. Worst-case scenario, you're providing your enemies with a fifth target who'll attract some of the combat attacks that would otherwise be coming your way.
It's not even successfully ticking its fantasy boxes. In ep.9, an old dwarf says of Rein, "I can't believe I'm being comforted by a boy not even half my age." Rein's a teenager. Tolkein's dwarves lived for about 250 years, with the longest-lived being Dwalin at 340. Mathematically, "half" is an understatement.
Similarly, there's an important bridge in ep.3. "Stride Bridge is the only way to travel from the Central Continent, where Horizon is located, to the Southern Continent." Are we supposed to believe that these people haven't even named their continents? Could this be any more generic?
The show avoids fanservice and potentially dodgy situations, sometimes to the point of farce. So, for instance, the girls in his harem who adore him and all sleep in the same room as him will squabble among themselves for the right to (wait for it)... have him tousle their hair. Yes, the hair on the top of their head. Naturally, the noble Rein has no sexual desire at all and is probably smooth featureless plastic down there. (The only exception to this is the "bathing in the lake" scene in ep.8 and that has magical light censorship beams to keep everything broadcast-friendly.)
Mind you, I did notice that Rein's harem members tend to get younger and younger. Also, the eagerness of all superpowered hot young girls to form a "contract" with Rein is open to all sorts of interpretations, very few of them good. Husband? Employer? Slave owner? (The latter would be in line with quite a few inappropriate isekai/fantasy harem anime of recent years.) Alternatively, Rein seals his contracts with blood, so should we start reading him as Satan?
In fairness, some of the girls are fun. The end title sequence is cute. There are occasional episodes when the show lurches into something vaguely resembling life. Otherwise, though, it's garbage. You immediately know how any episode will play out as soon as the current antagonist proves to be young and female. She'll be recruited and fall in love with Rein like everyone else. Welcome to the harem! The show's title sequence is a massive spoiler in this respect, except that a sixth harem member sneaks under the wire in the final episode. It would be hard to make this show lazier, either in the writing or the art.
...but it's quite fun. It's relaxing and nice. It's easy to watch. You can see how this kind of thing has spread across modern anime like a plague, even if this is an unusually blatant example... but it's also easy to see why lots of watchers (e.g. me) are describing it with words like "plague". Also "infantile", "sub-amateurish" and "his facial expressions make him look like an Auton".