Its very easy to understand why EH elicits such a guttural negative reaction from EMfags and YHfags.
For EMfags, its the quintessential female cuck scenario. Mikasa serves as the easy self insert at the start of the series for most female fans and for several years EM seemed like a guaranteed thing. That is, until the cutest blue eyed blonde starts spending time with Eren, and much more meaningful and impactful time, than the former I might add. The scene with the jealous Mikasa at the orphanage cements that in canon, Mikasa and her self inserts are being replaced by an objectively better girl. Historia also being portrayed as the Stacy cheerleader in the highschool AU only further makes this situation more personal for the Mikasa self-inserts, which lets be honest, are probably not all that popular irl.
For YHfags, Historia essentially rebounding from Ymir to Eren, and even having a child with him, hits home to the core insecurity of every lesbian: Being dumped for a man, possibly for personal cravings to start a family, something a lesbian couple can't do on their own without jumping through a bunch of hoops. Isayama discarding Ymir and killing her offscreen so he could start developing Historia alongside Eren for future plot lines that still haven't been revealed yet(Lol farmerfags) hurts.
Im confident my assessment of the real psychological layers of EH butthurt is fairly accurate but feel free to add on to it.
