Well, it basically fills the role of most damaging HA bow, at least that is what I always thought.
The problem with that is the accuracy penalty on horseback, the accuracy penalty to higher PD (3 higher than Nomad which is quite a blow), and you can only get to HA 3 at Lvl 31 because you need to put 18 into strength and it takes 6 agility per horse archer point. That makes for a poor shot. The slower missile speed does give you a better speed bonus when charging, but gets no better or worse when circle strafing other mounted ranged or pursuing cavalry. The slower missile speed also compounds that poor accuracy problem. Then there is the cost.
With ragged outfit, leather boots, Nomad and regular arrows, and a spiked club with an Arabian, I just break even on gold. The only thing you can lower in that setup is the Arabian. The courser wont be enough of a savings to keep from bleeding gold, so the best you can go with is palfrey or desert horse. Palfrey is slower than a courser with the same maneuver, so you can't get away from cavalry after you and with maneuver that low, you get eaten by HX and other HA. The desert has good enough maneuver, but the speed is so slow the maneuver won't help you against any of the other mounted classes either. Bottom line is, you go broke or become a slow moving duck.
That is just doing a strict HA compare. Throw in the HX into the mix and now you are really in trouble. Their accuracy is far better than your and they can hold it forever without the reticule expanding. You have to rely entirely on timing. Sure, if you can disrupt their reloading, you have a chance, but your accuracy is so bad from the causes listed above that you may as well hope someone else shoots them and disrupts their reload. At any rate, the armor they wear is superior as well, and with cut damage you aren't going to be one hitting them. They will one shot you though. So to compete, you need to upgrade from those arrows to bodkins. That's another 10k gold you have to remove from your equipment (5k x 2 stacks). That means you
might be able to afford a Rouncey, which again lacks speed and maneuver both compared to what others are riding that will be hunting your head. Oh yes, you are also giving up
18 arrows while also being inaccurate at the same time. At least you can have 6 PS since you won't need more than 3 riding with that Rouncey you will be shot and lanced off of every round. But if you want any wpf to swing it with any usefulness, you will need to give up some of that wpf in archery, which makes you even more inaccurate if you do.
You might as well be a ground archer at that point, and then the Tatar or Horn are better choices.
Can Yumi be the most damaging of the horse archer bows? Slightly. Can it be the most effective of the horse archer bows? Not from my experience and the numbers above are why.