Well, speaking for myself I disliked both of them immediately, hated them rising to warchief, and came to resent much of their respective fanbases for constantly deriding me as “red alliance”.
I suppose the difference is that when Mists was happening it was clear from the beginning that Garrosh was bad news. Sylvanas though? Even mid-mustache twirling Blizzard was insisting she was a heroic figure. As a result her fan base were significantly more aggressive than I remember Garrosh fans being.
“Traitor” is something I got called a LOT. It got extremely frustrating. I’ll admit to a sense of grim satisfaction when she finally revealed herself and I could once again unambiguously represent myself as Horde without having to use “Honor Horde” or “Saurfang’s Horde” or get screamed at for being an “Alliance bootlicker”.
Granted it could also be recency bias.