Stop start is nonsense in my mind and just a ploy for emissions
It depends on how it's done.
I think I've stated in the past Fiat's attempt is crude and cheap.
It utilises the starter motor and battery like a normal start, with a sensors to protect the battery.
It's basically self defeating, using battery power to restart that it then has to work the engine to put back in it before the next stop, which it rarely manages.
This wouldn't be too much of a problem if it had the opportunity to recharge, but in city conditions of repeated stops and starts, the sensor calls time as it's not getting the time to recharge.
You've probably nailed it on the head regarding emissions ploy.
On the other hand, I've a Citroen E-HDi with stop start and it works perfectly as some development and cost has been spent.
It works on a hybrid alternator and capacitors.
As you brake to stop, the alternator charges the capacitors, not the battery. It actually kills the engine just before it rolls to a stop, which is a bit disconcerting to start with.
It then uses this power stored in the capacitors to spin the alternator, not the starter motor to restart the engine.
It's really has stored and used wasted energy to perform the restart, it's also saves the battery from repeatedly heavy hits on the starter motor so isn't constantly working to recharge the battery between stops, it's win win.
It's pretty seamless and smooth, it never fails to stop once the temp gauge starts to move off the stop and restarts are instant when you foot leaves the brake pedal, so there's no pause as it restarts, it's started just in time without that awkward pause.
Due to it stopping the engine before stopping, I wasn't a fan at first, but it's gown on me now and really is saving me fuel around town.
At the moment I'm just over 3 mpg worse in to town than on a motorway run, my Panda TA was move like 16 or 17 mpg worse.
Shame it's a Euro 5 and will soon be banned from the city centres!