The only redeeming quality of this entire series is the OP. It's masterfully directed, has great use of music, and displays all the animation quality you're looking for in this series.
Outside of the OP, the other 18 or so minutes you spend watching an episode is entirely wasted.
There's no real story aside from fighting and characters can't even be called characters since they solely exist for fight scenes. I know next to nothing about any of the characters and feel no attachment or emotions towards either the demons or the idaten, likely because there is no attempt made at character development.
I'd say the most emotion-stirring scene so far was the totally uncalled for and random rape scene at the end of EP 1. I'm fine with dark stuff like this being alluded to or included in media, including anime, so long as it serves a purpose. There was really no purpose for the scene to be directed as vividly the way it was. Case in point is how EP 2 highlights humanity's cruel treatment (including the aftermath of the same rape from EP 1) towards each other without going to the same lengths as the randomly included scene at the end of the previous episode.
While the animation style is quite unique and the colours are vivid, I wouldn't say the fight scenes are even anything to get hype about unlike other pure action series; at least GOH had really good fight scenes. The fight scenes in Heion just feel like a Youtube demonstration of how to animate a fight scene. You know those stickman fight animations you'd watch as a kid? That's basically Heion but rendered in higher quality and you're watching it while on LSD.
So if you're just looking for vivid colours flashing on the screen to boost your adrenaline, you'll enjoy this series. If you're incapable of shutting off your brain for 18 minutes and just watching flashing lights, you're not going to enjoy this series. It's really as simple as that.
