HA is not underpowered, it is just difficult to play effectively. Regardless of your bow/arrows choice, your shots will NEVER land where you aim. It took me a long time to adjust to HA aiming compared to foot archer aiming. In third person as a foot archer, you just have to aim your crosshairs above your target and factor in distance. As a horse archer, you need to aim to the left or right as well, to account for your momentum. Even with 30 ping, I see my arrows released "behind" my horse rather than where I am shooting from. The height difference between mounted and unmounted is also a factor, but much easier to adjust to.
As for builds, I am 18/24 (no PS, no melee WPF, no athletics). 4 HA skill is the absolute minimum if you want to shoot farther than a throwing weapon can reliably hit. I took 18 str because I love strat battles, and would hate to be useless because I can't use the 6 PD requirement bows that the army has for equipment. 8 riding means that you can outmaneuver and usually outrun anyone, even skilled enemy lancers.
I use the +3 Yumi and +3 Arrows in battle/siege. The upkeep with an arabian horse is low enough that you won't lose money every round. You just have to pick targets with light mail or lower armor. Or horses (they are point piñata if you are ranged cav). Taking down enemy lancers/melee cav then galloping away from the dehorsed player, saying "[N]o" is funny every time. With bodkins you can shoot any enemy you see (still excluding shielders), but you get less arrows and pay drastically higher upkeep. It's a luxury option in my opinion. Just practice practice practice until it clicks! It's worth the effort if you really want to play HA. Remember, if you have equal WPF your horn bow or yumi will draw and shoot faster than the enemy's longbow/rus bow. Don't panic, just bump and shoot your trampled target.
Some maps on battle (and 99% of siege maps) are no good for horse archers. Even with 0 athletics and 0 ps/melee wpf I do better as foot archer if the map has nowhere for you to ride your horse and take advantage of it. I still prefer pure HA because this build requires the heaviest investment of points/stats. In my opinion, it's better to maximize HA capability than to become a mediocre horse archer and a poor melee fighter.
