The first thing that jumps out at me about these rules is the allowance of auto-hits on a stationary horse's hindquarters. Auto-hits of any sort are suspect to me. In a realistic melee setting, it would be nigh impossible to block or avoid such an attack, yes. However, even mild powers might include force shields around the entire horse to foil such attempts, and mounted fighters endeavored to stay mobile in order to not give foot soldiers any more opportunity to equalize the battle than absolutely necessary as well as present a more challenging target to mounted opponents.

Regarding attacks that target the rider, two of the most basic mounted combat feats in the D&D 3.0 system allow for the rider to drop behind his mount and use it as cover in the case of an attack headed his way to gain a substantial bonus to his Armor Class, and to cause attacks aimed at his mount to miss if he can make a Ride check greater than the attack roll result. These two benefits cannot be combined, however. It's a perfectly fair trade to sacrifice your mount if you're in good shape to continue the duel, and if you're hurting, you probably want to have your mount take weak attacks and/or help you get to a more advantageous location, particularly if archers are trying to pick you off.

Finally, jousting armor is considerably heavier than barding worn on the battlefield in much the same way that American football players wear heavy padding that soccer and rugby players lack: They are designed for maximum protection during short, explosive bursts of activity. One must also take care to prevent undue sores and chafing caused by putting armor onto such a beast. As usual, it's a trade-off between mobility and protection.