Great quote:

"When you get a demo and something works 90% of the time, that’s just the first nine. Then you need the second nine, a third nine, a fourth nine, a fifth nine. While I was at Tesla for five years or so, we went through maybe three nines or two nines. I don’t know what it is, but multiple nines of iteration. There are still more nines to go.

That’s why these things take so long."

Importantly, the first 9s are the easiest.

If you need to get to 9 9s, the 9th 9 could be more effort than the other 8 combined.