To be fair, we don't really know much about how Dahj was built like and how exactly she operated... she might have been a fully biological artificial human or she might have had a Terminator-like metal skeleton inside for all we know... we didn't really see the acid go much deeper than her skin. If she's cybernetical to a degree, she might have withstood explosions or acid dissolving her organic outer parts. But then again, considering she has a sister, it makes sense that the Romulans wouldn't have a problem with terminating her once she's activated and became dangerous. They still have the backup. Luckily, Maddox was savvy enough not to fall into the good old 'one single prototype' hole.