My thinking behind this is as follows ...

The associated coordinate from the dataset I was testing with had no units. As a result the netCDF loader defaults the units to `unknown`, which is a sensible thing to do.

Having a `sigma` with units of `unknown` will cause the final derived coordinate to have units of `unknown` ... which I guess we could just let that happen; that data is not fully described, so that's a data owner problem right? 

Given the CF spec, the coordinate should have units that are dimensionless. I suspect in user land, it'll be a very common case where no unit has been used to describe `sigma`, thus resulting in derived coordinates with units of `unknown`. At that point the user will care (if they notice) ... or not.

It's a grey area for me, so I punted for being graceful. Happy to back it out if you consider it against the Iris philosophy.