There's nothing unbalanced or Oped about mounted combat (with any weapon type), especially if it was its own skill.
Righto. It could be quite easily balanced by:
1. Making 'Horsemanship' it's own skill. If you want to get nasty at killing from the saddle, you have to put the time in - the skills you use on your feet won't be enough.
2. Making Horses vulnerable. This not only balances mounted combat, but makes Horse Armor actually valuable.
3. Making Horses expensive to purchase and maintain. This, combined with greater vulnerability, would make you think twice about every potential encounter. Also, horse thieves.
4. Horse Psychology. In addition to PC skill, make it so that individual horses have be trained to endure the stress of combat situations, otherwise they might spook and buck the rider at the first sign of trouble.