I have no clue about music in general, but ffdp are pretty basic when it comes to their song structure, a lot of simple and repetitive rhythms. Add to that that they sound recorded better than live (IMO) and they're American which can lead to prejudices in the metal community and the bands easy to joke about.
Their music was pretty great back in the day, but their more recent stuff sounds like schizophrenic whinging, and their music videos make them look like maga fascists. I have done zero research on what their opinions actually are.
I like their first couple albums and they write extremely catchy songs, but their lyrics feel just a little too douchey for my taste. They're a fantastic cover band though.
i think the reson behind that has little to do with the music. idk how it is now but a few years ago the band had baddd infighting and made it all public. i am talking up to the level of dissolving the band on stage, just to continue for the revenue the next day. I assume many of the ffdp haters were fans of the music back then, but got dissapointed by the musicans to the point they couldnt enjoy it anymore.
Remember if someone hates a thing, its likely they still care about it.