I find couching to be the easiest way to ensure a kill when riding through a group of enemies. Just press 'X' and ride through and you're almost guaranteed a kill at very little risk. If you're charging head on, or if you notice a spearman looking at you, just veer off and find another target rather than risk the counterstrike. The Eastern Steppe Lance is my go-to, with 200 reach.
Swinging polearms around like baseball bats on horseback is what requires more suspension of disbelief IMO. And it also makes it too easy to slice and dice your way through dozens of infantry.
But I'm sure we'll see the devs fine tune all the equipment in the game at some point, and this will probably include finding the sweet spot for lance length.
As far as the Bayeux Tapestry is concerned,
here is a commentary by R. Allen Brown on the issue of overhand strikes I found. From pg 12:
The assertion that Norman cavalry were little more than javelineers seems to stem from Richard Glover's 'English Warfare in 1066' and is refuted by other historians. There are other sources I've found that say similar things from other credible people. I'm no expert myself, I'm just quoting what I found on Google Books.
The passage referenced from D. J. A. Ross is
here, but is in written in French, and I can't read it and am unable to find a translation.