A highly dedicated Chainsaw Man fan took issue with the anime adaptation from MAPPA. Among his woes with the adaptation included the main protagonist's voice actor -- Kikunosuke Toya -- alongside the director, Ryū Nakayama his vision for the anime was vastly different than the manga in the eyes of the fan, and it lost a great deal of charm that it could've had.
It's also apparent that this Japanese fan isn't the only one who feels the anime wasn't as good as it could have been. Some fans of the manga found Nakayama's version to be unfaithful to its source material. It also doesn't appear to be doing as well in terms of sales as expected, which is a little surprising considering all the hype and excitement surrounding the anime before its release earlier this year.
Some fans also pinned a bit of blame on MAPPA itself, thinking that a studio like Studio Trigger would've been a much better choice for an action anime like Chainsaw Man.
It's also worth noting that the manga's creator, Tatsuki Fujimoto, was in close contact with the animation team and didn't want the anime itself to be too close to the manga as some of its expressive nature would be a bit uncomfortable or tacky in animation. He also wanted the team to have freedom in its production. Really, it sounds more like he wanted the barebones of Chainsaw Man to be followed while letting the team go nuts and make what they felt would be the best play.