Many Mega Man fans have their own strict (dumb) idea about what constitutes a "real" Mega Man game and what doesn't. Usually this boils down to fans accepting the platformer series (Classic, X, Zero, sometimes ZX) as "real" while rejecting the action RPG series (Legends) or turn-based RPG series (Battle Network, Star Force) on the grounds that those "aren't real Mega Man games" or something idiotic along those lines.
Or it'll be an issue of art style, with Classic fans saying
Mega Man: Powered Up is shit or not real because of its cute, chibi art style.
There's also the usual group of fans who claim Capcom "owed" us
Mega Man Legends 3, and while I was as heartbroken as anyone by its cancellation, they never "owed" us anything. They're a business, and businesses exists to make money. If I was an indifferent party in charge of Capcom's finances and looked at the facts surrounding the decision to cancel it (3DS had little to no traction at the time,
MML 3 is a sequel to a game that came out a decade prior and wasn't exactly a big seller), I'd have made the same decision.
Those same fans often liked to blame Capcom of America or its (at the time) community figureheads like Seth Killian for
MML 3's cancellation, when any rationally-thinking person of course knows they had absolutely nothing to do with it.
So yeah, there are lots of reasons Mega Man fans are awful.

For me, if it's got a badass blue robot on the box and is made by Capcom, it's a Mega Man game no matter the genre. Battle Network and Star Force are awesome in ways Classic and X aren't, and vice versa.