Basically, the longer the weapon the farther forward the enemy needs to be when you release the swing. If you've got a shorter weapon, like a mace or axe, swing when they're almost beside you and you'll take the head. If you're using something long, like the war razor (or other weapons ~115 in length) you basically need to swing when the enemy is at your horse's nose or the shot will whiff -- which basically requires that the blade somehow passes right through them if they're any closer. It shouldn't be an optimal swing when they're that close, but it shouldn't be a complete whiff, either.
I did the full practice range when I booted the game up, and noticed that hit detection felt wonky then. After missing one of those clay pots with my backhand swing for the Nth time (backhand used to be my specialty in Warband, so I was pretty indignant about it
), I stopped the horse right beside the pot, and started experimenting with swings and angles.
The number of times that sword passed right through the pot without breaking it was absolutely ridiculous. Unless I swung so that I hit the pot while it was beside my arm -- with the bottom third of the sword blade, instead of the top third -- it wouldn't register the hit.
If the collision meshes are that wonky everywhere else, I'm not surprised people are feeling it's off. I've adjusted and make it work in game now, but only by tossing logic out the window.