Oh, wow, okay, I'm going to have to be a HUGE pain in the butt here, but this is kind of bugging me.
When the characters change sides, their sprites so far are just mirror imaged. I noticed this because Nightmare is supposed to be left-handed. When a character like Saber is mirror imaged with their sprites, it's no big deal. It looks like she's just turning around. But for asymmetrical characters like Nightmare, it presents a problem, because only one arm is mutated, his right arm. So when he switches sides, not only is the left-handed Nightmare suddenly holding the Soul Edge in his right hand, but suddenly his left arm is mutated instead of his right.
The reason it's a pain in the butt for artists is it means they will have to draw an entire new set of sprites for when the character changes sides. This could also present a problem for characters who are not only asymmetrical, but even those with asymmetrical outfits. Personally, I wouldn't worry about outfits too much, people like me are going to nitpick on even smaller things. I just think it's important for Nightmare because of his mutated arm.
Also, although Siegfried is shown to be left-handed at neutral, his fighting style also shows him changing stances from Side Hold to Reverse Side Hold, so he can be perceived as ambidextrous because he can hold his sword in either hand. Siegfried's body itself is symmetrical, so I'd see no need to draw a new set of sprites for him because everything is already properly explained that he changes hands sometimes with his fighting style. But for Nightmare, it's almost a must for his mutated arm.