When a card is both considered a unit and a development can it be removed by a card that only targets one of those archetypes? For example, Stunty Smasha removes a development. If said development is also a unit, is it destroyed? My guess is yes, since it carries the advantages of being both, it should also have their drawbacks, but I would like to be sure.

Correct. The condition is that the target is a development. It doesn't matter what else it is. A unit that is also a development (or vice versa) can be destroyed by Stunty Smasha, just like it can be destroyed by say Wight Lord.

Yes, that makes sense. Many thanks!