Hmm... good question.

Yes, it depends on `units_func` so it's possible to fail in any way imaginable. But even with just the current aggregators, there's already one circumstance where it could fail: VARIANCE squares the unit, which I don't think is possible with logarithmic units.
```
ValueError: [UT_SUCCESS] Failed to multiply Unit('lg(re 1 meter)') by Unit('lg(re 1 meter)') 
```
If we want to make that nicer then perhaps the units_func should be responsible for catching the error and re-wording it. Thoughts?