[fwungy]

Efficiency means minimizing the use of the costliest components. It's installing fault points into a system on purpose.

Robust systems minimize fault points. Efficient systems come at the cost of robustness, and vice versa given a fixed definition of what is being conserved, i.e. costs or energy.

For example, a four cylinder engine that gets 15mpg will have a longer life than one that gets 30mpg, given the same cost.