The spread formation is a even better counter against mangonels for HA than other units, and that’s due to range+mobility. So they can take one mango shot, easily spread due to mobility, deal some damage on the mangonel, walk in, shoot again and most likely kill the mangonel. Other option is just to quickly ride away. If damaged after some area of effect hits, the Warrior Monks can heal them, and that’s a staple unit for Castle Age Rus.
I like the points Khorix made, above. They actually lose to archers not due to mere stat and cost comparison, but actually due to battle formations. As archers can clump up more, the battallion can fire more arrows than the HA. BUT, and that’s a big “but”, mangonels are a common thing in Castle Age. If both sides have magos, Rus have a gigantic advantage not only due to archers having less HP, but if the battle becomes mobile, while the battallions try to dodge the rocks, Rus have an easier time dancing through the chaos. I understand the design choice of mobility being an asset during battles, not only for map positioning, not only for kiting, but said asset shines a lot against the one unit (mangonel) that was designed to wreck big clumps of spammed units.
Besides, it’s absolutely true that Rus have a waaaaay to easy time to get into the advantageous position of Fast Castle into unit spam. They don’t even need to go for Horse Archers, if they want. HA are chosen due to being way to good for what we pay for them and their training time. It transforms the match, for your opponent, into an uphill battle. I mean, this can happen, but is way to easy for Rus to create said situation. WAY too easy.
So, in summary, HA have their problems, which, for me, are mainly the low price and high attack speed (bug or UI mistake? I don’t care), but also these two game problems make Rus’ life way too easy: Overtuned scouts (tanky, high torch damage for their role, can animation cancel) and uninteractive Pro Scouts. I believe fixing the problems mentioned in this paragraph can force variety into Rus’ comps, which is already a start that can steer the next balance changes into “buffing what is weak rather than nerfing what is strong”, which the devs said is what they rather do regarding this type of topic.