Fair point. Presumably some experimentation would be the order of the day. FOR SCIENCE! I'd be particularly interested to see what happens to materials which aren't available at all on one side of the portal, such as materials which were magically blended or completely artificial. There are apparently ways to pass magical items through the portal unchanged, given the Element of Magic and the copy-to-copy book Sunset Shimmer has, so presumably there could be at least a certain degree of trade in magical artifacts. We know that information can pass through the portal back to Equestria, so perhaps there would be ponies there interested in human technology, or at least in designs which could be recreated using the Equestrian industrial base plus magic? They might not be able to mass-produce microchips, but I'd bet they'd have a fair chance at producing nearly anything up to about 1980s-era technology.