Shams ad-Deen as-Sakhawi (d. 902 AH) wrote the biography of his teacher Hafiz Ibn Hajr al-‘Asqalani (d. 852 AH). In it he records Ibn Hajr’s reply to a question regarding al-Nasai’ and others’ criticism on Abu Hanifah, the Imam.
“And he was asked about what an-Nasai has mentioned in his work, “al–Du’afa wal Matrukin” regarding Abu Hanifah, may Allah be pleased with him, that he was not strong in hadith and made many mistakes and errors along with being scarce in narration, whether it is true. And whether any of the great scholars of hadith has agreed with this or not?
He replied: an-Nasai was one of the leaders in hadith sciences, whatever he said was based on his knowledge and ijtihad. (But) there is no one all of whose opinions are to be accepted. Some of the hadith scholars have agreed with the gist of an-Nasai’ opinion on this issue and al-Khatib al-Baghdadi has gathered their sayings in Tarikh al-Baghdad, of which some are acceptable and some that are rejected.
(As to the less number of his reports) it has been answered that it was because of his opinion that only such a hadith can be narrated which has been (perfectly) memorized from the time of its hearing till narration. For this reason (i.e. the strict condition) reports from him appear less in number otherwise he narrated many hadith reports.
In short: It is better not to get into this kind of discussions because the Imam and the likes of him have crossed the bridge (i.e. they are above any criticism) therefore no one’s (critical) opinion will be effective regarding them, rather they are on a degree to which Allah -the Exalted- has raised them in that they are followed and imitated. So trust this submission. And Allah is the Owner of the Success.”
as-Sakhawi, Shams ad-Deen, al-Jawahir wa al-Durar fi Tarjama Shaykh al-Islam Ibn Hajr, Dar Ibn Hazm, Beirut, 1999 vol.2
 The expression used is “crossed the bridge” which means whatever people might have said about them is not to be given any consideration. (See al-Sakhawi’s Fath al-Mughees, Maktaba al-Sunnah Egypt ed. vol.2 p.21)