Well, it's expensive compared to kero/LOX. It's expensive stuff to make, and corrosion and safety requirements make it more expensive to handle. When things go sideways, it's just nasty, as in the Nedelin disaster and that wayward Chinese launch that took out a small town.Despite that, hypergols have some good properties like room temperature dense liquids with decent Isp that autoignite. By itself, hydrazine is a fairly good monopropellant. So the stuff will likely continue to be used in satellites and spacecraft where the quantities aren't very large. But even there, new mostly non-toxic monopropellants are being developed with better Isp. I think the real original driver was for largish ICBMs that didn't have to be tanked with LOX, and safety was a secondary concern. These also made for good launch vehicles, so the legacy persisted even after liquid ICBMs were mostly retired.We certainly won't see fluorine; it was seriously considered and everyone chickened out on that stuff. There's toxic, and then there's crazy.Most countries and programs are definitely moving away from using hypergols in launch vehicles, and towards cheaper LOX, RP-1 and now, methane, as well as hydrogen. I think direct and indirect costs are the real driver, but safety is certainly a factor.
From the right launch site, would UDMH/NTO rockets be commercially feasible with only minor detrimental effects on human health?