It is a realy effective tactic, at most on the large, long maps. The matches on Fish Market are easily and usually depends on dismounting, because there are perfect field of view to any of the bases. Dismounting works with any damage, you can deal 1 damage to the enemy and still dismounted it. I don't find this tactic fair, because its too ez to do. It work total otherwise in the real life: riding people can also withstand hurts. Add some base resistance against dismounting would be great: make this tactic harder to realise and make the game more realistic. Dismounting haven't got any counter yet.
My idea is add 50% damage reduction while riding and a 15% threshold: You have to lost the 15% of your max health (fall to 85% health) for being dismounted. You can be healed while riding but can't use skills without dismount yourself. I also think that the horse should have much lower acceleration, slower controll and higher momentum as in the real life! It could have different animations with the different speeds. And dismount yourself if you ride into a barrier.
And add a defensive item to buff this threshold:
Heavy cavalry: Increase the threshold of dismounting to 30%/45%/60%. Basic cost: 150
I have much more new item ideas in an older thread in the same subforum for keep the 'same amount of items in each category' unwritten rule.