chloroquine is not frequently used for autoimmune.
At doses used for corona, it is within 50% of margin, or even lower if you are a slow metabolizer of an LD50 dose.
plaquenil on the other hand does have a higher therapeutic index.
The other major disadvantage is that these drugs can interact with protease inhibitors.
Kaletra, a protease inhibitor, is a combination of two drugs, one of which is a CYP3A4 inhibitor. It is used with some success on coronavirus patients.
Co-administering this with chloroquine 500mg/BID could easily result in chloroquine concentrations well above LD50.
plaquenil on the other hand does have a higher therapeutic index.
The other major disadvantage is that these drugs can interact with protease inhibitors.