Sorry, debihuman, I've been working my head off on a project, so I've gotten maybe 7 hours of sleep in the last week. I tend to get a little offensive when that happens.
It's just that... well, mathematically, crit fumbles punish the people who make the most attacks.
I.e., the group that needs the most help.
This is like not houseruling the monk to be proficient in their unarmed strike; it's piling unneeded penalties onto a group that doesn't really need them.
I can see why someone would use them, but, again, they are mathematically a poor idea.
And the difference between crit fumbles and sundering, is that you can build to avoid sundering (it's an opposed attack roll. And then, iirc, an opposed strength check), whereas, depending on how your ruling crit fumbles, they might be entirely unavoidable.
I started playing in 1e; I know what a cruel and capricious number god feels like, and crit fumbles give me flashbacks (to the system in general, not specifics.)
Sure, a bad DM can do worse; Shatter would work wonders in this situation; however, bad DMs usually seem to gravitate towards crit fumbles for some bizarre reason (apparently to balance out critical hits, which the auto-miss already does), which is why I called out "giving them ideas" as a bad thing. Because that kind of rule, when misapplied, already ruins certain archetypes (called most of melee.)
And it's a 20d6 max fall damage, not 10d6, lunar2.