Back when they first got really insanely famous due to Guitar Hero III, a lot of people thought their music was fake.

They were called studioforce by guitarists who felt threatened by their inability to play TTFAF, and those same haters justified their hate by pointing out that the band themselves frequently couldn't play it live.

This was due in part to the band still having a janky live setup at the time, in part due to their singer not sounding as good live (when they switched singers to Mark it made a big difference for their live shows IMO), and of course because Sam and to a lesser extent Herman would drink excessively before playing a lot of nights, leading to lots of recordings of their sloppy drunk shredding being uploaded to YouTube.

But mostly, I would argue, the hate came from people who dislike new things. They tried to invalidate the band's obvious technical prowess as a way to disregard their (at the time) relatively innovative sounds.

You see that a lot in the guitar community. Anything new or that doesn't fit a traditional, pre-existing mold is dismissed as inferior. Anything non-traditional and obviously skillful tends to draw hate from those same people.

Now that Dragonforce has been around for a good long while, the traditionalists have had time to adjust and Dragonforce has had their live act consistently together for well over a decade, the hate has largely died down.

It also helps, IMO, that Herman Li does regular livestreams so people can see that he's a relatively down to earth guy, especially for someone with a giant wall of guitars in his house and a collection of high end sports cars.