Talking about cadet pilot places, Aer Lingus had a success rate of less than 0.5% last time it ran it’s program. BA Future Pilot Program was around 2%. Lufthansa Academy less than 10%. It is a very small number of applicants who are actually selected to start training.
In my country, Australia, it looks like airlines running cadet programs have a selection rate of 1-5% of applicants. For the most part the individuals chosen all graduated from their courses, with a maximum of 1 or so trainee not making it through on a course if any, but this isn’t a regular thing.
Civilian cadet courses seem to select applicants whom they are very sure will pass, as opposed to military flight training that seems to start a lot more applicants than they expect to pass.