Because once at a set of lights it decided not to re-start the engine when the lights changed, missing a complete set plus anoying everyone behind while I manually restarted. Granted this only happened once but is enough to sow the seeds of doubt.
Because although very good in most circumstances, I find on occasion it cuts in at just the wrong moment. The delay it causes is small but noticeable - and if you wanted to go for a gap gives you second thoughts.
It is active in 'Dynamic' mode... not great when you roll up to the lights for a quick get away and because you've pressed the brake pedal just a fraction too hard the engine dies meaning your get away is interrupted (again a small but noticeable pause).
Basically, IMHO, it's good but any tech that should be unintrusive that is noticeable probably isn't doing it's job right...
It's the most used button in my car which isn't right.