I've been following my 7 y/o play on Wii U & PS4.  He's pretty good playing with a regular controller, and beat SM3DW with the Wii U Game Pad, so I told him to try Mario Galaxy 2, which uses the Wii-mote + Nunchuk.

He's been having a much harder time getting used to pointing and aiming with the Wii-mote, for example controlling Yoshi to hook onto things, or eat and shoot stuff.  Progress is slow and clunky compared to using a regular controller, and he's dying a lot.  So I was thinking maybe some use of Wii-mote may have made games less accessible.  What do you think?

Easy to pick up, hard to master.

I don't have any young family members to compare with but some older members do seem to struggle with basic pointing at times... their natural inclination is to extend the arm to try point, and it's much harder to aim accurately when you have three joints to control (shoulder+elbow+wrist). Meanwhile I can aim perfectly as i tend to rest my wrist on something, or if the environment doesn't allow that i'll hold my arm against my side.

Having said that, most of those family members struggle if there are more than 3 buttons to press, and apart from my sister, my dad is the only one who can use a control stick... and only with his right thumb... which means conventional controllers are far worse.

^ sure, well he's played a few days with it and knows what to do with it, but it takes pretty long to get through some levels.

This. Has he played Nintendo Land? You would probably have to explain how some of the minigames work to him.