Any elite selection process has high risk for not selecting the best people.

Either due to corruption or, even more likely, institutional biases.

Malcolm Gladwell has a few episodes of revisionist history that looks at some US universities who fall into this trap.

Even with all that said; the chances of a selection process for 17/18 year olds being able to correctly select the best future adults in their field is very low.

Another good example; the number of young geniuses or prodigies (whether maths, chess, acting etc.) who don't make anything of themselves.

Oxford & Cambridge are very selective and entry is very competitive. Unusually for UK universities they hold a separate entrance exam for all courses. (Information about the extra admissions tests done by some universities and some courses: https://www.ucas.com/applying/before-you-apply/what-and-wher....)

Fun fact: When I applied to Cambridge I was interviewed by Mary Beard.