<<If they have been a contributory factor in incidents, then why are we bothering with trials?! If they have been proven unsafe, then ban them. >>
I do not believe that conditional clearances are unsafe in any way, shape or form. What may be unsafe is the way controllers issue them and crews respond to them. A conditional clearance issued correctly, with due emphasis on speed of delivery, especially to foreign crews, with careful attention to the read-back and eyes outside the tower window, should be as safe as any other ATC instruction. I was pedantic in the extreme when issuing them but I know that some controllers gabble instructions at warp speed, which is asking for trouble. Slow down.... take an extra second to issue the instruction, listen for the read-back and watch what's going on and there should be no problems..