Smoothness is not related to the number of bearings. As an example to disprove this claim, the Zenoah G-62 has bearings on both sides of the crank throw. It is not a smooth engine, at least I've never seen one that could be described as smooth. The ZDZ-60 has roughly the same displacement and 2 bearings in front of the crank throw, just like all of the glow motors. The ZDZ is also much smoother than the G-62.

Things that do effect smoothness are reducing the reciprocating weight (of the piston and con rod) and choosing the right mass of crank counterweight. Still, there is no way to achieve anywhere near perfect primary balance with a single. Boxer twins are much better from a dynamic balance standpoint, but they also will have a secondary vibration as a result of the offset cylinders.

A big factor that is often overlooked is the mixture distribution in the engine, the mixture setting (rich or lean) and the ignition timing. In a 2 stroke, if the mixture is too rich, then the motor doesn't fire every revolution, and the engine shakes quite a bit. I have seen some pretty bad vibrating boxer twins as a result of too rich a needle setting or possibly fuel puddling in the crankcase. A 1/4 turn of the low end needle on the carb can often really smooth out a vibrator. My stock 3W-100 used to vibrate a fair amount, and could not be smoothed out with tweaking the needles. I compared it to a friend's 3W-80, and his engine seemed like an electric motor. I finally added the carbon intake horn to my motor and there was a dramatic change in the smoothness throughout the rpm band. My motor is now as smooth as the 3W-80. The only thing that changed was the mixture as the motor came off idle.

Having never seen a Taurus, I don't really know if they are as smooth as claimed. I can offer up a theory; they might be smooth because Stallspin knows how to adjust the carb properly and does this on every engine before he ships it out. The other manufacturers rely on the customer to adjust the carb. Vibration is a funny business, with lots of claims, but no hard data for comparison. Lots of anecdotes.