In an open field with no obstructions cavalry charge should be damn scary and it certainly worked in history as well. Of course, most people in history learned NOT to give such an easy target.
The Battle of the Golden Spurs is a perfect example. The Flemish knew that a French counter-attack to their siege was likely and found a nice flat field... and proceeded to riddle it with ditches, drained nearby rivers to make marshland and even dug pit traps covered with branches and leaves. The French arrived to find the flat field they had expected crossed by several small streams and tried to put bundled branches over the streams to let the horses cross more easily but the Flemish were guarding against that and stopped the French attempts to fill in the streams.The French felt they HAD to give battle even in the unfavourable situation and lost badly.
Agincourt the English constructed wooden barricades and earthworks as well as using the already existing marshy ground.
At Crecy, the English were at the top of a ridge and again dug pits to slow or stop the French cavalry and had the unplanned extra benefit of a strong rainshower just before the French attack which led to heavy mud in the recently ploughed fields in front of the hill as well as small rivulets of water draining down the ridge making the French attempts to climb up the now slippery slope quite a bit more difficult.
At Castillon, the battle which ended the English attempts to claim the French throne- the English attacked into the French prepared positions but it was the cavalry charge by the Bretons across a clear field with no English defences that sealed the victory.
Bannerlord already has plenty of battlefield diversity which can break up cavalry charges. If an enemy deploys right in the middle of a field with no defences they should be overrun by cavalry if not screened by their own cavalry.
As it stands melee cavalry are barely any threat even to unprotected low tier archers.
More important then obstruction is to have flanks protected. Which is exactly the case in all those three battles. And it's also reason why universal anti cavalry infantry formation all through history was square (sometimes circle). That is formation that does not have flanks.
Cavalry have only one advantage over infantry: mobility. Thanks to it's mobility, cavalry can exploit weak points like flanks, or even rear. That's what makes it dangerous. If cavalry can't exploit flanks, or there are no flanks, it have to try to create weak point, usually some kind of crack in infantry formation. But that's very hard to do against half decent and determined infantry.
Also mobility gives cavalry initiative. Thanks to mobility it's cavalry that can decide when and where to attack. Besides tactical advantage that it gives, it also huge moral disadvantage to the infantry. Because infantry can't run away from cavalry while cavalry can from infantry. Just one mistake and it's over. Just one man that break the rank and cavalry can exploit the gap and whole line will collapse. So infantry really needs to hold it's nerve under charge. And again, that requires experienced men.
On the other hand stationary infantry is a sitting duck. Man fixed on a huge vulnerable and easy to spook animal with limited ability to fight and defend himself. Cavalry also can't stand as close to each other and create as dense formation as infantry can. So there are less men fighting per unit of distance.
Take mobility out of cavalry or at last limit their mobility, by not presenting flanks for example, and there is not much that cavalry can do.
Of course this is something very hard to simulate in the game like this. It would require much more advanced AI and morale system. Therefore we will only have some simplification. Right now cavalry is much more balanced then compared to Warband.
Against the most heavily armored targets, arrows should do very little.
And so should swords, axes and spears. But then you would have no way to kill heavily armored targets, would you?
People forget that armor and combat in general are abstract in Bannerlord. When you hit armored target with the arrow and do damage, it does not mean that you have pierced through steel plate. It simply represents % of shoots that hit unprotected, or lightly protected areas on the target.
So armor definitely does not need any buffs against arrows, it's effective plenty enough.