I am an FIC authorised Instructor - and my expeiences of FI(R) students who have passed the ATPL exams & flight tests are as follows:
Many have no idea whatsoever about basic physics - eg's: don't know the difference between thrust & power; assume that forces, componts & resultants are all synonyms etc, etc. However, they have all been given enough practice papers to pass the exams - possibly without any real understanding?
A considerable proportion of what is taught on approved ground-school courses is 'simplified' - eg's:
Coreolis force - only ever explained N/S. That's because the conservation of momentum arguement falls flat on it's face E/W - and for good reason - because it's wrong. Sad, because the real explanation isn't really that difficult;
Compass errors - have you ever noticed the compass card 'dipping' when flying N or S? - me neither, and that's because it doesn't - oh dear - another wrong 'explanation';
Lift - the usual diagrams miss out the most important part - the starting vortex! Can you believe that the vast majority of the worlds' professional pilots have no idea of the basic principle that keeps them airborne? Unfortunately they don't! - it's the effect of viscosity.
But it doesn't stop there - now we come to flying training. CPL students are taught to fly in a commercially expedient fashion, which is sometimes inappropriate to SPPL instruction eg. - it is not appropriate for a 10 hr first solo student to lower the flaps in a final turn, or to use the power - roll - pitch recovery when the a/c stalls from the inevitable pitch-up. Yet many CPL graduates assume that what they were taught on their course was 'gospel' - having not understood that the techniques they learned for the CPL were appropriate for a higher level of experience.
In conclusion I can see no reason why a good PPL should not be allowed to teach to PPL level without passing the CPL/ATPL exams and flight test, provided the FIC ground-school is carried out properly.