Although I am supporting the attitude that "It's realistic" or "it's historic" is not an argument for things which would break the balance, I think that "It isn't historic or realistic" can indeed be an argument against things which don't have anything to do with balance.
Why would someone use a Flamberge on horseback when he is only able to stab with it? I guess those swords weren't really viable for stabbing at all, because their blade was not stiff enough most likely. I don't have any facts for that, but it's how I imagine it, because from what I saw those Flamberges often had a blade which was rather "whobbling" or "bending", similar to the rotor of a helicopter.
But I do agree if some other two handed weapons should be usable from horseback, with the needed balancing of course, because it was historically accurate. You can see knights wielding two handed swords on horseback
here for example.
