First, viability (versus feasibility) tends to be what’s riskiest or hardest for most products.

First, viability (versus feasibility) tends to be what’s riskiest or hardest for most products. So yes, you should start there. But that’s not always the case…

If you are trying to sell a trip to Mars (or cure cancer, or …), it would be prudent to de-risk feasibility first to some level — making that the riskiest thing…

Next, I never said anything about “building” the hardest thing… but “testing” the hardest thing. Building can be slow… Testing doesn’t have to be.