The theory of critical hits

[CharPixie]

A few things to consider:

Overkill is more likely with critical hits (especially the 4x style ones). So, wasted damage is more likely.

Critical hits have no effect outside of a small range of to hit numbers. So, statistically, they will produce far more outliers than the normal, predictable damage dice (or even the simple threshold that's the "to-hit").

Until a certain level, criticals can often mean instant down/death when at full hitpoints.