As a longtime user of ancient Macs, I find the modern iMac's longpress-powerbutton frustrating. It usually takes me multiple tries to figure out if I pressed it long enough, or too long, or not enough. When I have pressed it just right, the iMac still doesn't respond immediately in a visible or audible way: there's just enough delay for me to start fiddling with the power button again.

Compare to the momentary buttons on ye olde Macs that existed on both the case and the keyboard, which gave immediate feedback. The one on the case also had a longpress action, but only as an override.

"Nope, didn't want this on right now. Bumped it by accident. Long-press!" is easier for me to get behind.