One addendum: The last line comments that the DRI can still maintain lift with the two lower wings. There is further evidence of this in Lothar von Richthofens account of haveing his top wing blown off in combat. Lothar, despite loss of lift and no aelerons, made a semi controlled crash and survived.
The flip side to this is that Lothar exaggerated a bit (big surprise
). His claim that his top wing was blown off is not entirely true, as evidenced by the fact that it is still very much there in a crash photo. However, there is IMHO no reason to doubt that his plane suffered serious structural damage that resulted in lift loss and complete loss of aelerons (he used rudder to bank).
Anyway, if it was up to me, here's how I would model it:
Structural failure without combat damage would be rare, and would only happen in rare circumstances. In the case of both the N28 and the DRI actions were taken to fix these issues, and for the most part (but not completely) these fixes were effective.
However ... I would make these two types more prone to serious structural top wing damage in combat. It should not happen every time you hit the top wing, only if they take leading edge damage.