For Senti specifically, the nano-and femptomachines used by most Yamataian medical technologies. are preyed upon by the Senti immune system symbiote, causing blood infections.
The same key that makes them nearly immune to most normal viral infections and even the high viral load required for infection of most illnesses can be easily handled without downsides. More often than not, these bacteria break down the complex proteins of cell waste, prions, viral shells, nanomachines, whatever, into fructose and sucralose for the body to use. This can even be helpful in dealing with venoms, as the enzymes will be surrounded by small bacterial colonies, preventing them from acting in their designed capacity. With large doses of viral load such as a hemosynth injection will cause the symbiote to reproduce out of control and cause massive opportunistic blood infections.
The other downside is that while Senti are bioplastic and are capable of switching to an iron hemogloben in stead of their normal cyanogloben, they do have hemogloben recycling organs, which will nearly immediately break down the oxygen carrying capacity of the hemosynth and turn it into more food for the symbiote.
Combined with their oil based blood solvent and the saline used for most injectable drugs, and we have a recipe for cardiac arrhythmia as the saline breaks down the fluid continuity of the blood. Combined with the osmosis properties of water, the saline will dissolve vital components of the blood, causing clots and coagulation, even if you can solve the cardiac arrest.
TL;DR: Hemosynth is horrifyingly lethal to your senti crew.
