In the case of high winged planes and high positioned upper wings there's the drag couple to address too. Dive the cabin model trainer with limited elevator control and the extra speed piles up the wing drag in a hurry. Add in the extra drag of the negatively loaded lifting airfoil and you get a LOT of high centered drag moment holding the nose from tucking under properley. And with soft responding trainer types there may not be enough down elevator force to totally counter that drag along with the rest of the factors.

Add enough down elevator area and deflection and it'll overcome that but it will never be happy.

I have a Fleet'ish Barnstormer that is marginally powered and it will NOT do an outside from a dive entry. But flip it over to inverted and push down and it'll go around just fine for the first half but the second half is a different story. I did it once by cutting the power to idle and the big 4 stroke prop held it back enough to just squeak around. All the rest were 3/4 outsides to a dive and positive pull recovery.

Semi symetrical airfoil here (low camber and thick in other words ).

Astrohog. Your buddy with the miniplane may have been fighting a couple of factors. One his airfoil may have been like mine. Two, he may not have had enough down elevator. Three, his model setup may have had quite a lot of incidence in the wings and or tail. To do decent ousides you need to be fairly close to 0-0. And finally all the rest of the setup may not have been enough to counter the speed induced high drag couple or at least it may have been just enough to make the elevator's job too hard.

But it's certainly not a "bipe" thing. Ever seen an Ultimate fly?

At least that's my theory and I'm sticking to it....