Think about it this way. An engineer's job is to think of all things that could possibly go wrong and which is why we test and do math to come up with a solution. Well, here's a good example of why I am concerned about the engineering behind the start/stop components. My GTI has 34k miles. I had the water pump break on me causing my engine to leak coolant. Me, as an engineer, I would try to make sure that the water pump I'm designing can withstand the pressure, heat, cycles, and all other physical forces that heated coolant will create. Seems to me like the engineers at VW didn't do too well with the calculations/design for the water pump because the cracked water pump has happened quite a few times, although not enough to make it a recall. You'd think they would have gotten water pumps 100% correct since they are a technology that has been used for 50 years or so. But that's not the case. There's been other recalls for other parts in cars and such because of mistakes that most of the time can be traced to either engineering or manufacturing.
Also, as an amateur house mechanic, I have seen low quality OEM components in modern cars that are inferior in quality than comparable parts from older cars that maybe more heavy duty. Just like interiors, they've gotten extremely cheap in the last decade.
So with all the recalls and trivial problems, it tells us not everything is 100%, so is start/stop perfect? On paper, yes. Real world, not sure yet we gotta wait and see how long these engines will last. As I said before, starting a car requires a hell of a lot more than just a starter, which makes me wonder how reliable it will be 5-10 years later when cars have over 100k miles.