The weakest point should be the nylon bolts so that they shear before pulling the guts out of the fuse. They will stretch when you apply torque to tighten the wing. They will snap if you carwheel the plane or plant a wing (I personally know of such things [:'(]).

I'm with you in your thinking that reinforced (with tri-stock) 1/4" ply with embedded 1/4x20 blind nuts epoxied will be more secure. You gotta decide if you can do a competent "upgrade" given the design of the plane's guts. If you can't then the nylon threaded bushings are as good as it gets.

I think that you'd have nightmares of stripping the threads out of those lil black thingies ...