A controller is not the same as an instrument, it generates a control signal that can be used to control the actual instruments (synthesizers, drum machines or a well hidden laptop). And I have built both controllers and instruments and have written a MIDI implementation so I got a bit of an idea about those things.

And if you tell me a mechanical engineer made his own instruments for his good sounding one-man music act the most boring, least exciting, least innovative outcome would be him adding some knobs and buttons into a cool looking enclosure. If that enclosure would look less cool he could very likely still make the same well sounding music, meaning it is not essential to the sound, meaning it could be seen as a prop.

I don't argue this is verboten, it is just a bit disapointing that is all.