No "challenges". Your shifting just works opposite.
I ruined a high normal (Suntour) when I had a chain break and wrap around the FDER, turning it into a pretzel.
I installed a low end Shimano and it worked fine.
I think the idea was a "good" downshift was considered "more necessary" than an up shift. Pulling cable is more positive than spring tension with a gunked up cable/DER.
"Rapid Rise" RDER's also shift "backwards".