So I thought about it on the way home and it dawned on me why I was having so many issues with Link swinging his sword in the wrong fucking direction all the time. So I just tested it out and HYUP
The problem is I have a tendency to snap my wrist at the beginning of a lot of specific movements. This is probably 100% from practicing martial arts where your two main power sources are your hips and your wrist (those big beefy muscles on your upper arm barely do shit but get your hand from point A to point some fucko's face). Unfortunately the motion controls are set to basically register any sudden jerky movement as a slash, so the moment the wrist snaps the game goes "OKAY YOU CHOSE TO SLASH IN (some random cockeyed direction)" and ignores the rest of the motion.
For the curious: the snap for me comes any time I need to change the orientation of the blade. For some reason (which is probably incredibly obvious to actual fencers) I always want to do a left slash underhanded and a right slash overhanded, and of course vertical slashes are vertical.
Now whether snapping like that is a good idea in real swordfighting or if you're holding a Skyward Sword as Enlarged as Link is something I don't know and also kind of fucking irrelevant because I don't want to learn a new martial art just to play goddam Zelda. But there's a whole gamut of little player behaviors like that out there*, and it kind of takes a step towards explaining why people seem to have such a polar "Holy shit this thing is garbage" vs "I don't see what the problem is" experience with the thing. I find that if you treat the controller like a wand or a small knife or a joystick floating out in space, you're golden. Get too fancy with the thing and it gets confused, which is a shame, because that's not a function of the 'mote itself, that's the game's designers dumbing the system down to a discrete set of simple and obvious choices.
* Which, now that I think about it, was probably thoroughly tested but not internationally - the Japanese are heavily invested in a particular kind of swordfighting form in which odd behaviors like mine would most likely cause you to cut yourself in the face.