The su and sh alleles are on different chromosomes. When combined, they produce flour corn, instead of sweet corn. It is possible (with several years of effort) to select for strains that have compatible alleles to produce sweet corn. These varieties are called "synergistic".
In my own plant breeding, I will not use the sh2 allele, because it produces seeds that lack vigor, and fail to thrive. They are only viable with heavy application of poisons to the seed.
I minimize the use of the se allele for the same reason.
The problem with breeding for "all known resistances", is that we can't possibly know what that would entail. There are resistances to soil, farmer's habits, sunlight, clouds, humidity, aridity, viruses, microbes, fungi, bugs, wind, mammals, birds, reptiles, etc, etc, etc. If we inbreed a crop enough to say that it is resistant to every known thing, then it becomes an industrialized variety that is doomed to failure when something unknown comes along.
