True, however that's AVRs and not Adruino. You very quickly get out of arduino-land when you have any constraints like battery life or performance. By far the vast majority of people contemplating either Arduino or RPi will never know enough to be able to tune things like that.

Heck the last time I looked into doing sleeping on an AVR-based Arduino it basically said go look at the 328's manual to set the registers directly. (Although that was a long time ago.)