I remember my first encounter with critical hits and fumbles, way back in the days of AD&D 2nd edition. Back then I enjoyed the idea of killing a monster with one hit if I got lucky or breaking my +1 long sword if my luck was bad.
Now I realise that they are NO FUN. For the most part anyway.
Why, you ask, are they not fun? Here are a few reasons
1. They are unfair to the players. Yes, cutting off a goblins leg is fun but having a PC you've played for years gain brain damage isn't. The goblin was meant to be killed, but your PC was meant to last.
2. They take too much time. Most of the tables for these rules are either expansion or home brewed rules hidden in binders that the GM must dig out. Then rolling and interpreting the chart takes a while. All this takes out the rhythm in a combat.
3. It steals narrative control from the players and the GM. I like describing combat, both as a GM and a player. These charts don't let the one who did the critical hit or miss come up with a description why their actions were so successful or miserable. And isn't that one of the keys of roleplaying?