I believe the major problem is that you do not get a seniority. It might be that you remain as a boy pilot for several years.
Otherwise for e.g. the SAA boy pilots(S/O) on the 747 seem to be very happy - good status and progression.
The Cathay freighter way could be the better choice, although they most probably do not interview guys with the min. experience as stated in the ad`s.
In the wannabes section you will find some stuff about Cathay.
Cheers