I don't know about it being a big no-no in boxing. It's a legit technique, just perhaps overused and with questionable form and setup when used in MMA. It might be the most natural to throw and generate power from though, given the amount of time that most MMA fighters can afford to train their boxing. Most people grow up around football/dodgeball/baseball, so some of the muscle memory is probably there to begin with. The superman punch is similar to the overhand right in this regard as well.
As for Fedor, his style of punching isn't exactly textbook boxing. He often seems like a wild puncher, but it's actually his style to do so. Fedor was surprisingly technical, in his prime especially. He throws something called "Casting Punches" (This is off the top of my head, so bear with me) it originates from Sambo (One of the two, anyway) and before that, supposedly from studies and research from the Soviet Union, back when they were doing a lot of experiments on human physiology, one of which being optimal power generation in the human body.
The issue with this sort of punching technique, however, lies in the fact that the whipping motion means it is often difficult to be accurate with a Casting Punch. Fedor had numerous injuries, where he would hit end up hitting his opponent with his thumbs, dislocating them numerous times. It's not a stretch to say these accumulated injuries contributed to his decline at the end of his career.
Also forgot to mention: they also have a tactical advantage of being able to parry someone's guard down, presenting openings, being difficult to track, and often used them to set up his Sambo grappling techniques with great effectiveness. People used to consider him the ATG for a reason.
