>OO classes make use of private functions that may be incomplete or faulty implementations of functionality available in other third-party libraries used by the same project. It isn't clear exactly how any function works in a given class relative to similar functions provided by other classes.

In C, at least, it's also possible to prevent others from using a certain function. So, if I wanted to use the functionality that function provides myself, I'd have to write a function with that functionality myself, which may be an incomplete or faulty implementation of that function.

>In a well-designed, totally self-sufficient, fully-encapsulated object, I suppose it wouldn't matter, but how many classes realistically provide all the functionality you'd ever need from it without having to subclass it at some point?

How is that any different from having to write your own function because a certain library doesn't provide the functionality I need?