HA is a great support class and you can do alot for your team. Much of this support can be made with shitty damage and accuracy like dehorsing enemy cav. Alot can even be made without shooting like taunting them to chase you around, have archers focusing on you instead of your teams and so on. If you play alone as a HA it takes alot of experience to be somewhat effective and you need to be one of those above average players to do well. To be one of the heros of the team as a HA you need to be outstanding. It also depends on what you try to achieve I guess. Staying out of harms way shooting at random ppl just to get high score or hunting peasants for kills is probably easy but you're not helping your team that much.
However just like archer squads are easier to coordinate in focusing on one target compared to melee squads, groups of HAs have an easier time than melee cav. And since cav is more maneuverable than inf, cav has an easier time choosing its fights or in military terms, seize the iniative. That makes squads of HAs both easier to coordinate and deploy compared to any other class IMHO. I wouldnt go as far as saying that a HAs effectivness in groups is exponancially but the curve is steap compared to other classes. A few HAs and maybe one or two melee cav focusing on the same target = endless arrowstuns, bumps and melee attacks. Very effective and rather safe on the giving end but very annoying for the receiver.
IMO Alone the class is rather weak maybe even UP but just like Gurnisson said buffing it might make it too powerfull, especially when you coordinate your attacks.
As for the hate I think its a combination of HAs being cowards (smart) only attacking when the opponent is at a disadvantage having a hard time (being unable to) fight back. And that its common knowlage within the majority of the community that its ok, even mandatory, to say bad things about the people who play HAs. Sort of hating a minority to be accepted by the majority kinda thing. Oh and one more reason is that when a selfish player goes HA he have the means, and tend, to delay, bully peasants, attentionwhoring, trolling, "killsteal" and being a douchebag in general.
