i agree, too.
all specials gone makes fighting very boring.
can you still cast while mounting?
i see reality problems too on headbutt, knee and kick, and there might be a balance issue that explains the denial of all specials in mounted combat, but i think there are more interresting ways to solve that, if.
maybe a wielded twohander is a bit of a reality problem, too.
so i ld suggest, as others before, a headbutt fail if the enemy is too small, all leg specials disabled, maybe (for balance resaons) one hand for the mount and one hand for the weapon, and additional mount special(s) like a mount bodycheck or a charge in direct combat, which is realistic.
the issue mounted combat leaves a lot of additional variety and is generally fun, which seems now more reduced than developed