First, please explain what BIW means. A google search didn't clarify.
While it would seem logical that Tesla should do a better job with each car they bring to production, the reality is that they don't. They always think this time it will be easy and there is always something to bollix up the works. Since none of us are on the design team, we won't know until they are making the cars if they have done a good job of preventing problems or if Musk has dictated things (like falcon wing doors) that are likely to create problems for a high volume production car company.
Then I will say, the body seems to be the biggest difference from the 3. There has been criticism of the body design which some say makes the car more expensive to manufacture than it should. This is something Tesla has to get right if they are going to become a properly profitable company. Profit of only $139 million on revenue of over $7 billion is not going to let them do the things they are saying they will do.
With volume model Y production being almost two years out they have time to get a factory up and humming, but if they aren't making the model Y in volume (meaning something around 5k per week) I won't be surprised. I think Wall street might and hit them hard. At some point the "innovator" and "disruptor" labels won't buy them forgiveness they have received in the past. It will simply become a matter of delivering on their plans.
