C++ has things like the fragile base class problem meaning you can accidentally break it easily. There's issues with throwing exceptions across different libraries on some platforms (maybe just Windows?) but I forget the reason why.
However, when the discussion already mentions "ABI contracts", they're probably referring specifically to the "fragile binary interface problem" (especially regarding member field access), which does not affect all languages that offer inheritance.
As the Wikipedia article mentions, this more specific problem is (confusingly) often referred to just as the "fragile base class problem".
p is short for predicate.