@ZombieGenesis My experience has been that having a staff that's willing to say 'no' to things that are clearly out of bounds for what's reasonable for a given character is enough to prevent cases like what you're describing. Harley 'has' taken a punch from Superman and not died, but Harley is pretty obviously not actually a brick on the level of a Kryptonian, and any charstaff would be well within their rights to just say 'no' and let the player process that as they will.
There's nothing wrong with using a system of some kind - whether it's loose and informal, like the numerical benchmarks HeroMUX used, or an actual game system like M&M or FASERIP - but I would disagree that it's the only way to manage players wanting to push characters in these ways. There are definitely players who will make problems of themselves when faced with those kinds of limits, but - like you point out - this can and will happen with numbers, too-- and numbers introduce a risk of 'power creep' over time, if the staff isn't careful about maintaining consistent standards.
This can get trickier when trying to compare characters who genuinely are in a similar niche and at a similar level of power, of course. Thor, in the last ~decade or so, has pretty regularly been written at a level of power that'd be at home with a Kryptonian character; should he be as strong/invulnerable/etc as Superman on a MUSH? Should he be slightly weaker because he's a powerful weather manipulator on top of the strength? Do you account for the fact that there's no real Kryptonite equivalent for him? That kind of thing.
(For the record: I would probably lean towards 'yeah, whatever; he has other flaws, and at the end of the day, there can always be something that disadvantages, distracts, or otherwise prevents him from being some kind of nuisance in the course of play, even if it's just a really fuckin' big robot. Also, if he's a dick about it, nobody will play with him, same as any other character.)
There will always be some people who try to push the envelope, codified system or no. I feel like you would have to take the extra step of having a system and also taking away players' ability to set the numbers in that system to have a real shot at curbing it (though, even then, they'll probably ask). That said, though, most players I have dealt with - and my experience is definitely not deep or expansive, but it's what I've got to run on - will just take a 'no' and revise accordingly. I did dozens of apps on HeroMUX and can count the number of players who really dug in and gave me shit about being asked to tune their stuff to be more reasonable on a hand.
@Auspice @Alamias
During my last stint of charstaffing, I had a 20-something Damian Wayne cross my desk who insisted on having pretty broad sorcery abilities, enhanced physical capabilities, and either full or partial control of the League of Assassins following a dip in the Lazarus Pit.