What AQHQ knows about the odds of success for a operation are and what some shmuck is told about the odds of success are two different and unrelated things.
Even granting that the bombers are gullible and unstable, and that "AQHQ" will oversell them on the chances of success, they can see the actual track record, and the mere act of training them sensitizes them to all the ways things can go wrong.
There's also a tradeoff between their naivete and effectiveness: a bumpkin who's never traveled internationally might believe whatever his handlers tell him, but is also more likely to draw suspicion or otherwise foul the mission.
There's also a tradeoff between their naivete and effectiveness: a bumpkin who's never traveled internationally might believe whatever his handlers tell him, but is also more likely to draw suspicion or otherwise foul the mission.