My DD went through the school system 40 years behind me and it has been interesting to compare her experience.
I taught GCE and CSE and then saw through the conversion to GCSE. GCSE replaced a very unsatisfactory and inequitable system.
Only a fixed percentage of candidates passed GCE so each year students failed who should not have.
The exam papers were very predictable. It was possible to look through past papers and work out what questions would come up. (An experienced teacher could probably do the same now for GCSE but I suspect they wouldn't be able to predict questions word for word as often happened in the past.)
Students who were entered for GCE were picked at least two years in advance and, often, long before that when they passed the 11+ so there was little or no scope for the late developer to tranfer in.
GCEs favoured boys because girls tend to do better on coursework and boys do well on last minute cramming. (I know that's a sweeping statement but the subsequent surge in girls' exam success since the introduction of GCSE bears it out.)
CSE was actually a pretty good exam system. It allowed for more innovative teaching and much, much more interesting exam papers and it contained a large element of coursework. It was the best aspects of CSE that influenced the GCSE syllabi.
The fixed pass rate at GCE meant that pupils doing dual exams attained a Grade 1 CSE yet failed at O level. Yet the Grade 1 CSE student had often put in more work and achieved a greater understanding of the subject only to earn a certificate which had little academic recognition.
(A grade 1 CSE equated to a grade C O-Level.)
DD is far more fluent in a foreign language after taking it for GCSE than I was in the language I took for O level. For that matter, she and I both continued our language to A level and I never achieved the level of fluency that she has.
She studied equally difficult texts for Eng Lit as I did and, though she didn't 'like' English the way I did I find I have nothing to complain about when it comes to her understanding of the texts. Her knowledge of grammar is far greater than mine was at the same age.
Her strengths are science and maths and there we have little in common. DH, who is of the same mind-set, is in awe of the depth of her knowledge and very aware of how out of date his knowledge now is.
40 years on I can honestly say that she has had a better all-round education at a bog standard comprehensive than I had from a selective high school and her GCSE experience was far more stimulating than my time studying for O levels.