What are the arguments behind the relatively late dating of Khuddaka Nikaya and Abhidhamma Pitaka?

It is generally told that the Khuddaka Nikaya is the latest among the five Nikayas of the Sutta Pitaka. Similary, the Abhidhamma Pitaka is also considered as the latest composition among the three Pitakas. It seems that these are universally accepted truths among the scholars of Buddhism. But what are the exact arguments behind these two assertions?