These are photographs of my broken shower tap here at home by Moen. It was the 3rd model to break in 5 years. The parts are not carried anymore by Moen. Staff at Moen had pity on me and sent me the parts by mail the previous 2 times. I don't know what the designer was thinking when he/she created the part. The rounded shape of the inside provides bad traction to the tap. It always broke the same way. And the rounded shape of the faucet provided a bad grip to close the water it also. It's one of the worst product designs I've seen.
If I were to create a shower tap, I would try to provide the easiest grip possible. I would create something like this: