He gets "flak" from serious boxing nerds because he's the self-anointed greatest ever and his modern fanbase, who are gassed by viral clips of knockouts, have no idea how weak is resume is or how strong past greats were. Whether or not someone likes/hates him is irrelevant, anyone who thinks he's currently a "great" is clueless and anyone who thinks he's toilet is clueless.
The facts are - He's literally beat ONE man who could've been a possible top 10 contender in previous eras - Ortiz. A man who in this modern era of multi titles not only failed to capture a title but hasn't defeated a single former titlist.
It's not "hate" to compare him with former fighters and it says nothing about Fury or Joshua to speculate who would have beat him. If anyone thinks Wilder could have knocked out Oliver McCall you DKSAB and that's a fact. So therefore he has to win on points, which takes skill, something Wilder abundantly lacks, hence losing the majority of rounds against his two credible opponents.