Well I'd argue that the cubes shouldn't be concatenated together.

Consider the scenario where two cubes could be concatenated together *but* one is masked the other is not. What should happen in the situation where the ```fill_value``` of the mask is set to a value that is present in the other non-masked cube? Should the non-masked cube be promoted to a masked payload and those previous valid values masked? I don't think this is a behaviour we want.

Being simple and safe is the best option here. Concatenate like with like, and this applies to masked payload. Otherwise it gets complicated quick.