The biggest problem with LSD is its unpredictability, and what I think is the main reason why it isn't used therapeutically despite having shown very promising results.

You can't reasonably OD from LSD but you can have bad trips. Some of them can be so overwhelming that they will cause lasting effect not unlike PTSD. If you can stop a trip dead in its tracks, that's one less risk. By having ways to counteract its various potential negative effects, you can improve the safety profile of the drug, and therefore make it more acceptable.