HX, or the ones we believe to be overpowered are an extremely specialized build. The template, for 30, is generally 7/30. 7 STR to use a Light Crossbow, then the rest dumped into agility for riding, weapon mastery and horse archery. I think you have two points left over for Iron Flesh. However, you must take into consideration, there is no versatility here. Once dehorsed, a HX is pitifully slow on foot, and without a speed bonus, the Light Crossbow is not particularly strong and has horrible ranged. The HX can't melee, since 2 Power Strike would be worthless, and therefore not invested in. Unlike the Horse Archer, who is stiill slow on foot, the HX does not make even a decent foot crossbowman. The HX is useful in Strat field battles, assuming one side has Light Crossbows, Steel Bolts and Horses, but useless if they are absent. The HX has almost no use in proper sieges, while the HA suffers almost no penalties until the ramparts are taken.
Also, HX, like HA, is expensive. The cheapest recommended horse is 13,000 to start, if you're not going for the Arabian Warhorse which is 22,000 or the Eastern Horse which is 24,000. The Light Crossbow is 5500, and the bolts are 2500, usually used with two stacks. The armor is a bit inconsequential, however
Hybird HX, much to Asheram's chagrin, have recently received a pretty big nerf. They must now use 0 slot 1handers if they wish to keep a shield, or use the pathetic 0 slot shield.
I don't believe HX to be too overpowered. It is a highly specialized class, with extreme strengths and weaknesses. It is also much more expensive than any infantry class, besides those in full plate. It is nearly useless in Strat. The unarmed horse charge nerf affected HX pretty adversely, as well.
Most players think HX are annoying, sometimes even those on the same team. That annoyance comes from the play style that a HX must play in order to stay alive and be effective. However, I think the issues lie in the rules and admin enforcement. If the HX is not going to dismount and go for the flags at the end of the round, kick him. I've seen Rohypnol do this on both his HX characters a number of times. The admins let him farm his valour, while everyone waits around for another 2 minutes.