Why does having a base class disqualify a class from being aggregate?

Question

What is it about having an aggregate public base class (or even multiple aggregate public base classes) that would make a class lose the nice properties of aggregate classes

Answer 1

How would you initialize the base class?

Derived d = {{1, 2}, 3, 4};

Or

Derived d = {1, 2, 3, 4};

Is

Derived d = {3, 4};

permitted?

To avoid all of this, no base classes.