2h receive 30% speed & dmg penalty from horseback, so an already slow weapon like morningstar becomes super slow. Slow weapons also receive more speed bonus, so you can make up for a good amount of that loss. Slow weapon isnt necessarily a bad thing on horseback but you have to know the pros and cons.
Advantage is you can 'hang' your weapon out for much longer and basically clothes line people with it
Disadvantage is you cannot make multiple swings in a pass very well. Also if anyone gets near your horse, theres no way to counterswing and kill them, you HAVE to ride away to regroup.
IE...if my horse gets reared and someone starts chopping away at me, with a sword I can swing or stab back and hit them in the face to kill them. With morning star it swings to slow they'll hit you in the foot and interrupt your swing.
The only polearms that slash horizontally from horseback are the hafted blades, but the animations for them are so slow and short they're useless...its an exercise in futility using them.
Put the majority of your WPF in the weapon you use the most and get the most kills from. You can be decent cav lancer with 0 polearm skill, but up to 75-90 does help considerably. Over 90 is probably a waste unless you are a polearm user