Although....
SOs seem to get through alright. You never hear of them failing during the initial training. If they are going to get picked off it is usually later on , say year one or two.
We do lose 4-5 DEFO frieghter guys a year which would be 5-10% depending on induction rates. Mostly because someone figures they can't or won't fit the Cathay mold.
There is also the option of being given an SO position if you don't make the cut as DEFO Freighter, but they still like you.
Good Luck.