The problem with meritocracy is that in order for any true meritocracy to function, it must reward merit and only merit, not means masquerading as merit. In other words, there must be equality of opportunity. Yet meritocracy rewards merit with more opportunity, making equality of opportunity impossible at any point, but especially in the later stages of life.
The more merit you "have", the more opportunities you are given to gain more merit. Conversely, those who do not "have" merit are never given an opportunity to gain it.
What meritocracy really does is widen the opportunity (income, educational, etc.) gap between those who "have" it and those who don't.