Luck and Endurance can only be pushed so far before you have player disbelief.and raised two examples: two characters of 10th level falling from a 100-foot cliff, and a low-CON, low level MU healing faster than a high-CON, high-level fighter. Do these examples break suspension of disbelief?
A lot has to do with what the players are trying to believe in.
Let's discuss the first example in this post. The 100-foot cliff is a recurring argument and constant target of tweaking. Originally, if you fell 10 feet or more, you took 1d6 damage per 10 feet. Thus, if you fall 100 feet, you should take 10d6 damage: a minimum of 10 points, average of 35. Using S&W core for a moment, since my LBBs are still in storage: a 10th level fighter has 1d6+2 hit points per level up to 9th, plus 3 more points for 10th level; a thief has 1d6-1 per level up to 9th, plus 1 point for 10th level. An unlucky player who always rolls the lowest result on hit dice would have a thief that is guaranteed to die from a 100-foot fall, but the same player's fighter might have about a 50-50 chance of surviving. A lucky player who always rolls high would have a fighter who could survive any 100-foot fall, or a thief who would have roughly a 2/3 chance of surviving.
Does this break belief? If you're playing a gritty game, yes. If you're playing the opposite extreme -- high action -- no, although a lucky player of a thief would probably complain about a lucky player of a fighter if both characters fell off the same cliff. But then, in LBB, or S&W, having high hit points is one of the few class features of fighters; they don't get magic, there's no such thing as skills or feats. Fighters are guys who can survive 1oo-foot falls, or ten sword thrusts. Thieves -- well, thieves were always broken, that's why old school people debate whether thieves should even be allowed. Magic-Users have the same hit dice as thieves, though, and they have to trade the ability to survive a 100-foot fall for the ability to cast spells, possibly including a Featherfall or Levitation spell. That's how they are expected to survive that cliff.
If you're playing somewhat less than high action, on the other hand, and not using one of the tweaks out there, beyond a certain falling distance threshold, you're going to call for save vs. death, or perhaps a CON/System Shock roll, since the original rules say CON allows you to withstand adversity. Falling off a cliff is a little different than being attacked by ten orcs, so it's reasonable to treat it more like near-certain death than something you could potentially avoid.
For something grittier than even save vs. death, GMs typically go for one of the house rules. For example, if I were running a gritty game, I'd say something like every 30 feet of falling distance means one randomly-allocated critical injury of 1d6 per 30 feet. Thus, a 100-foot cliff would cause a base 10d6 damage as normal, but also three 3d6 injuries. I would just need a random body part chart, or more likely I'd roll on a dice map with regions named after body parts. I might also rule that a critical injury to head or ribs with a point value higher than CON requires a save vs. death roll; even on success, note that the character can do nothing until someone comes along to do some first aid, and afterwards the character will have penalties on actions.