a question about horse archer training

[BroskiDerpman]

I doubt horse archers were OP in EB especially how all the missiles got nerfed greatly compared to vanilla and many other mods. Then most units have vastly improved defense and many got huge armor buffs.

The one thing that did make them powerful in game was cantabrian circle.

If given the choice I would take 8 fast cavalry and the rest be Roman heavy infantry and just cover the whole map in a melee rush. (Phalangites are good too as heavy cavalry can't do anything head on against them)

[Arjos]

Iirc asymmetrical bows, with stiffeners, already existed in our time-frame. Some had been found in the Lake Baikal area, they had been termed Hunnic, but possibly already spread to other parts of Eurasia way before the 4th century CE...
BTW about stirrups, it is unclear whether the Avars imported them or actually invented them in Europe. Nevertheless leather loops or socks already existed in several places and EB's timeframe :)

[kdrakak]

Iirc asymmetrical bows, with stiffeners, already existed in our time-frame. Some had been found in the Lake Baikal area, they had been termed Hunnic, but possibly already spread to other parts of Eurasia way before the 4th century CE...
BTW about stirrups, it is unclear whether the Avars imported them or actually invented them in Europe. Nevertheless leather loops or socks already existed in several places and EB's timeframe :)

The Hunnic bow had been a big surprise for late Romans according to the sources, since it could penetrate shield and armor at a range that was previously considered safe. An asymmetrical bow allows for a longer draw which may account for this change in effectiveness. If it existed, I imagine it must have been a less powerful version or unknown in the west.
As for the invention of stirrups, lance straps and the like the they are all referred to as being of the Avar arsenal, which the Byzantines adopted. This does not negate what you say but it's the only written source related evidence that I am aware of.
It's hard to really know isn't it?