I always ditched the thumb controls....3 wheelers and the later quads.
It was too hard to gas w/the thumb and hold on w/the rest. It might be ok on something that is a wimp mobile, my machines always had big power and I needed my thumb and at least 2 more fingers (1 or 2 on the brake) to hold on.
I even converted my ski to a twist grip.
I laways thought it was a government issue. When some noob squid jumped on a quad and gave it a thumb full of gas, he'd actually let off and thumb-grasp the grip.
Too many others to list....