So basically the multi-taper wing is just a simpler construction method trying to approximate an Eliptical pressure distribution, which (generally) gives you reduced induced drag, making the airplane retain more energy through things like snap rolls, as well as when making pitch changes, either positive or negative. Additionally, as the chord gets smaller towards the tip, the wingtip itself generates less vortices, which again improves efficiency. The end result for pattern airplanes, is less power is required. For electrics, this can be a big plus in that you can get through a sequence using less battery. I think this is a big reason why you see BJ making comments that the Biside requires much less power than he expected. The induced drag created by the airplane in pitch changes is much lower than he expected as a result of the planform.

The big problem I see with these wing designs is keeping them stiff enough to retain their shape, and especially at the wingtip, ensuring you don't wind up with any twist, both statically (as built), and aerodynamically (under load). The static form is easy enough to verify, but aerodynamic, not so much.

Please note, I'm NOT suggesting the BJ Craft airplanes are having any aerodynamic twisting issues.