which also makes me wonder why Kanger even bothered with this ...
I suspect that twin-chamber tank has something to do with it not being glass. When you crack a Protank, it's pretty obvious, and you're unlikely to keep using it. And even if you did, so long as you don't take it apart, there's no chance of glass getting into the tube. With the Unitank, and its spring-loaded tube, it seems like you could crack the tank separator and end up inhaling tiny bits of glass.
Just a theory, obviously. Glass, with the two-chamber design, would be pretty tough to make to begin with (maybe an extra metal ring as a tank separator?), not to mention expensive, and the thing is already more expensive than a PT2...