carbs are designed to mix fuel and air, and throttle air, over the full range of operating conditions. Those conditions can vary a lot from bike to bike. When you tune by porting or adding a pipe, you need more fuel in the powerband, but you don't want to run too rich outside the powerband or you are all rich and blubbery (loading up) at small throttle.
Bings tend to work better on setups that need a progressive fuel delivery curve, you can tune the needle to run lean at low rpm/high vacuum/closed throttle, and still leave a large main jet to run nice and fat when you're ripping.
SHA's don't have the ability to control that as well, but they also tend to be more forgiving for bikes that have a fairly 'flat' fuel delivery curve.
I've ported stock cylinders that run really well on an SHA, they are also good on stuff like Peugeot and Tomos with the reed valve. For piston port, i almost always have better luck with a bing.
the worst part about the bing is the literally thousands of combinations possible of slides, needles, atomizers, jets, etc. Even some of the bodies have differences. If you're starting with a used pile of parts that may not have come out of the factory all together, you can really drive yourself bonkers tuning it. If you buy an off-the-shelf 15 for 100 bucks you might as well just get a VM18 which is better for everything ever period.