I will take some time to justify that title, but first, let's talk about what Back-Arrow is. A guy with amnesia claims to come from beyond the wall that surrounds everyone, though the idea that anything exists beyond the wall is heretical. In this enclosed area, two kingdoms are at war. Back Arrow, as the main character comes to know, gets wrapped up in the conflict as he tries to escape the wall.
The fights can be pretty great but require some explaining. The Bind Warpers people put on to make use of a person's' Conviction Particles' to form a giant robot type thing to fight. A Conviction Particle is a physical manifestation of a person's conviction, and that affects what kind of mech they summon. These convictions range from 'Don't Rope me into things' to 'Unrivaled' or 'I'll find a way' and no, I have no idea how these end up resulting in their abilities in the end. 'I'll find a way', for example, can.... redirect attacks sometimes? In short though, the stronger your conviction the stronger the power. Back Arrow, however, has no conviction. And because of this he can alter his shape or multiply. I can at least follow the logic on that one.
Beyond the head of one kingdom being power-hungry and kind of violent, I don't understand after twelve episodes what they are fighting over. By this point, the heroes have pretty much joined with one side because.... they aren't being assholes. The actual story isn't all that interesting, and the characters are all pretty simple archetypes. They are fine enough and nothing too egregious, but not enough to carry the story on their own. It means overall the review is going to be short, but the show just doesn't give you a lot to talk about that's directly relevant to the story.
That said, time to justify that title by bringing up the show this most reminds me of, Tengen Toppa Gurren Lagann. I'm not saying there aren't genuinely interesting or clever things in that anime, but there is no denying how stupid that show could get. The only reason that plot went forward is one character, defying any semblance of sense or logic, smashes his head into the problem over and over again, and he just got lucky that drive was what powers the mechs of that show. Eventually, the show grows to a point they slap legs and arms onto a giant boat, and that giant boat mech runs up a hill, jumps into the sky, and roundhouse kicks a plane. Gurren fully committed to how ludicrous it was that you're able to forgo the insanity and just go along with the ride.
Back-Arrow has a similar idea with convictions, though you have room for a lot of variety with it, and similarly it just kind of lets things happen for the sake of doing it. The problem is it never really feels like it commits to it. You have some bizarre things like a farm where one country is genetically creating pretty boys in pretty inhumane conditions (I don't know if this is supposed to be a commentary on the K-Pop industry or not, but it certainly feels like it), but that's just some oddity that is brought up one episode and never touched on again. It just feels like it won't fully commit to the absurdity of what's going on, and because of that, it's hard to just let it all sweep you away.
It would be one thing if the characters or story being told were interesting enough to get you interested, but it doesn't have that much to offer in that regard. Back-Arrow had a lot of interesting ideas to it, and some pretty good fights, but in the end, it just feels like a middle-of-the-road anime. Watch it if you have nothing else to do, but don't go in expecting too much or you'll be disappointed.