Between the activity required to control the mount and the cap on OB, it's hard to be good at mounted combat unless the character is dedicated to it. In order to help make it more feasible, I use the following rules:
* A mounted combatant has a +10 OB vs footmen.
* Riding bonuses for mount quality apply to Riding and Mounted Combat skill (this might be the way it's meant to work; the rules aren't entirely clear).
* Any melee attack made while mounted inflicts an additional Unblancing crit of two severities less.
* On a charge, the Unbalance crit is of equal severity.
* A trained warhorse will generally take intelligent actions if not controlled (eg, once a charge begins, it will keep charging; when in melee it will provide a stable fighting platform, and potentially attack unknown people nearby).
I think the Unbalance crits are based on a comment someone else made on the forums here at some point.