Well Chilean attempt - aircraft launched with reasonable guarantee of having somewhere to land. Chances of survival good. Risk - 3.
Pebble Island - aircraft launched with reasonable guarantee of having somewhere to land. Chances of ingress good and egress reasonable. Risk - 5.
Black Buck - aircraft launched with some guarantee of being refuelled as modest. Chances of attack and survival good. Chances of recovery or diversion low to medium. Risk - 7.
That is a very subjective summation of what was a very small part of a hugely complex operations. If you look in detail at each element for success/failure, as you do when calculating bombs required for a 2 sigma success, then it follows that then number of potential failure points in the Black Buck raid far exceed the number of potential failure points on the other two.