Aku no Hana may be the least accessible anime ever made. The visual design is intentionally unattractive to annoy viewers who are accustomed to conventional animation styles and, if they haven't dropped it yet, to stress the theme of finding beauty within the ugly. The narrative is inspired by Baudelaire's Les Fleurs Du Mal which for non french speakers requires a translation to even be read nevermind understood.
if the viewer finds these decisions to be pretentious then the show is unwatchable. There's no base level of attraction to pull you in unlike other esoteric anime with convoluted narratives - you can sympathize with Lain as a character, Texhnolyze is violent, Ergo Proxy has impressive production values even if the narrative made little sense. Here you either exercise extreme patience to understand or you don't, and for most it's really not worth trying to understand it.
It does hold value for those who are interested in Baudelaire, decadence/some ideas of aestheticism and will look past the surface to see the broader picture. I think it's a great anime if those requirements are met. but I wouldn't fault anyone for thinking it's trash because it's literally designed to upset viewers who don't fall into a very specific niche.