The biggest drawback to one of the vaccine candidates receiving an FDA Emergency Use Authorization: it would halt the Phase III trial and destroy any chance of getting more data about that vaccine’s safety, efficacy, and longevity.
On the This Week in Virology podcast, TWiV #661, guest Ralph Baric detailed what exactly is at stake if an EUA is issued early in a vaccine trial: it would compromise the original end-point of the study. We would forfeit the chance to learn about late-developing effects such as antibody-dependent enhancement (in which a vaccine sometimes exacerbates the virus it was designed to tame). This link starts at 31:12 in the YouTube video (emphasis added).
In a glimpse into our near future, Dr. Baric discussed the prospects for effective antiviral drugs (not vaccines) that might be given, orally or by nasal spray, upon a positive test for Covid-19. Watch for about two minutes beginning at 26:32: