In my limited experience building, a radiused corner tube will require any thru-bolts to be located further away from the adjacent tube wall by 1 radius, whereas a square corner tube will let the bolt exist .005" from the inside wall. The closer to the inside wall you can bolt, the greater the torque load can be because the tube wall is in nearly pure compression. Further away induces a bending moment at the corners. There may be other solutions for this I am unaware of. Also, bolts on both sides of the tube would be closer together (by 2 radii) in a round corner tube. Generally the farther apart bolts are the more secure the attached component.
I have seen photos of radiused tubes used in gyro construction but I do not know if any special internal reinforcement was used.