Comment by SAI_Peregrinus

I think the disconnect is that the impure portion often has substantial amounts of inherent complexity. If you're working at a high level that's handled by a lower level (OS, framework, etc.) but somebody is writing that framework or OS or HAL. And they need to test too, and ultimately someone is going to have to make a test double of the hardware interface unless all testing is going to require hardware-in-the-loop.