I've never used a crit chart or something similar, but if I ever use one it would have to:
- be as simple as possible: one or two rolls at maximum
- give a temporary penalty, not a permanent one
The second is IMHO more important than realism: no one would want to carry a permanent penalty, especially if it comes from an unlucky roll and not their own mistake (a scar is ok, a scar that permanently gives you -1 to all attacks is not). Furthermore in a game where you can resurrect the dead, it would actually be not very realistic if you couldn't reattach someone's limbs or heal permanent injuries.
Downtime between adventures can also be a factor to the design of such system. If you usually have lots of time between adventures, you can easily assume that "permanent" injuries are removed before the next adventure, so they aren't really permanent.
edit:
Thinking about it, two rolls is already too many, since you have already rolled (1) attack, and (2) critical confirmation roll.
If I use a system like this, I want only at most 1 extra roll, which would be needed to determining WHICH penalty. A d20 for example if using a relatively small chart, or a d100 if someone wants to get crazy with weird effects
