Reynolds Number, Prandtl number, power factor, efficiency, anything that is a fraction of full range, mechanical strain, etc.

There are many terms in an engineering project that are unitless. Having a system to automatically check the units on the remaining scalars is cute, but doesn't do anything to prevent you from improperly performing operations that relate efficiency and power factor by accident, for example.

Multi-input, multi-output control systems regularly assemble state vectors out of terms that have mixed units. You can easily have amps, volts, meters per second, and radians per second in the same mathematical vector.

I have yet to see a compile-time unit-checking scheme that adequately addressed those issues.