Well, Let's see, in beam structures, cylindrical beams are not as efficient weight/strength but also in torsion, particularly as the strength comes from the hight of the web, where the I beam shines and a tube has most of its mass in the center... (as explained in section modulus.)
Having said that, from an economics perspective, there is nothing like a cylindrical extruded spar (some call it an AL tube
) you basically cut to length and drill... easy to automate too. A Bearhawk is, well a different kind of airplane, and those spars take 300+ man hours to build, so I don't see the cost effectiveness there... if you where to compare it, then looking at the Maule design is a better solution form a cost perspective albeit a little more expensive to get started, as it cost some extra money to build the dye for an extruded spar, but its the same principle, cut to size and automate it on a CNC drill.
Randy @ RANS has done a good job over the years, and has all my respect as far as design is concerned, and this is possible as he makes business decisions that do not jeopardise safety in his designs.
I have been following his design for the S21, looks interesting, but I find it miss fortunate that he chose a D shape spar incorporated in the LE, this exposes the spar to any accidental contact... and in the bush this is quite common, and could leave you stranded. It does save you loads of time if your where building the wing though...
Happy holidays!