Speed Racer is doing two very difficult things to get right:
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be an adaptation in the first place: adaptations are hard. People have expectations from the get go and you have to try and meet those while putting a spin on the source material that these same people can either like or look past. Almost all of these wind up failing in the public consensus.
*
be a live action anime: this is probably just as hard if not harder to get right than the above. Anime fans are crazy about their animes and doing a live action anime and meeting the source material accurately in any way that isn't awful is insanely difficult, pretty much none of these ever work. They pretty much always fail to feel like an anime while also looking tremendously cheap/awful.
Speed Racer is not only a very good modernization of its source material but it's the only live action anime film I've seen that does its anime justice.
You may disagree or whatever of course, but I think this is the heart of why people who like the film like the film. It really does feel like a live action anime (ESPECIALLY the
final racethis is like peak anime), and it looks anything but cheap. The CGI still holds up extremely well.