Islamophobes are fond of attacking Islam with certain stories from Ibn Ishaq's account which are not authentic. There are however 3 criteria we'll use to discuss this but let's go straight to the point.

1- His original biography didn't survive: The original text of the Sīrat Rasūl Allah (Biography of Muhammad) by Ibn Ishaq (Medina 85 A.H.; Bagdad 151 ) did not survive. Yet it was one of the earliest substantial biographies of Muhammad. Fortunately, as noted above, much of the original text was copied over into a work of his own by Ibn Hisham (Basra; Fustat c.218 A.H.)Ibn Hashim himself claimed:"God willing I shall begin this book with Isma'il son of Ibrahim and mention those of his offspring who were the ancestors of God's apostle one by one with what is known about them, taking no account of Isma'il's other children, omitting some of the things which has been recorded in this book in which there is no mention of the apostle and about which the Quran says nothing and which are not relevant to anything in this book or an explanation of it or evidence forit; poems which he quotes that no authority on poetry whom I have met knows of;things which it is disgraceful to discuss;matters which would distress certain people; and such reports as al-Bakka'i told me he could not accept as trustworthy - all these things I have omitted. But God willing I shall give a full account of everything else so far as it is known and trustworthy tradition is available.[The Life of Muhammad, A Translation of Ibn Ishaq's Sirat Rasul Allah, with introduction and notes by Alfred Guillaume [Karachi Oxford University Press, Karachi, Tenth Impression 1995, p. 691]

2- Muslims Scholars Opinion: Shaykh ibn Taymiyyah said:"Allah has provided evidence (i.e. Isnad) establishing the authenticity or lack thereof of the narrations that are necessary in matters of the religion. It is well known that most of what was reported in aspects of Tafsir (commentaries on the Qur'an) is similar to narrations reporting Maghazi (or Seerah) and battles, promoting Imam Ahmad to state that three matters do not have Isnad: Tafsir,Mala'him (i.e. great battles), and Maghazi. This is because most of their narrations are of the Maraseel (without reliable companions or successor) type, such as narrations reported by Urwah Ibn az-Zubair, ash-Sha'bi, az-Zuhri, Musa Ibn Uqbah and Ibn Ishaq." (Shaykh Ibn Taymiyyah, Majmu' Al Fataawa, Volume, 13, page 345)Imam Malik on Ibn Ishaq:Imam Malik was not the only contemporary of Ibn Ishaq's to have problems with him. Despite writing the earliest biography of Prophet Muhammad, Scholars such as al-Nisa'I and Yahya b. Kattan did not view Ibn Ishaq as a reliable or authoritative source of Hadith. (Jones, J.M.B.Ibn Ishak. Vol. IV, in Encyclopaedia of Islam, edited by Ch. Pellat, and J. SchachtV.L.M.B. Lewis. London: Luzac & Co., 1971: pages 810-811)Malik bin Anas Bin Malik bin Abu Amir Al-Asbahi (715-801 C.E.) or Imam Malik-- lived cloest in the time to the life of Prophet Muhammad of all the collectors of the hadith (Bukhari, Muslim, Abu Dawud, etc). He was born more than 80years after the death of the Prophet. Imam Malik was a complier of a respected hadith collection, called Muwatta. Imam Malik was a hadith scholar. Imam Malik called Ibn Ishaq a liar and an imposter for writing false stories about Prophet Muhammad. Imam Malik has said that Ibn Ishaq"reports traditions on the authority of the Jews".(Kadhdhab and Dajjal min al-dajajila. Uyun al-athar, I, 16-7)In his valuable introduction Ibn Sayyid al-Nas provides a wide-ranging survey of the controversial views on Ibn Ishaq.In his full introduction to the Gottingen edition of the Sira, Wustenfeld in turn draws extensively on Ibn Sayyid al-Nas.Imam Hanbal and Iand Ahmad on Ibn IshaqImam Ahmad Ibn Hanbal was asked about the solitary reports of Ibn Ishaq if they are considered reliable. He said“No!”. See Tahdhib al-Tahdhib, Da’ira Ma’arif Nizamia, Hyderabad, 1326 A.H. vol.9 p.43Because of his tadlīs (unrealible sources) many scholars including Muhammad al-Bukhari hardly ever used his narrations in their sahih books.(A Biography of the Prophet of Islam, By Mahdī Rizq Allāh Aḥmad, Syed Iqbal Zaheer, pg. 18)According to al-Khatib al-Baghdādī, all scholars of ahadith except one no longer rely on any of his narrations, although truth is not foreign to him.(Al-Khatīb al-Baghdādī, Tārīkh Baghdād)Others, like Ahmad ibn Hanbal, rejected his narrations on allmatters related to fiqh. Al-Dhahabī concluded that despite his good qualities any narration solely transmitted through him should probably be considered as containing munkar. (Al-Dhahabī, Mīzān al-itidāl fī naqd al-rijāl, at "Muhammad ibn Ishaq)

What Non-Muslims says about Ibn IshaqThe work of Ibn Ishaq is much of criticism. Ibn Ishaq work contain several false stories regarding Muhammad. Even Western Non-Muslims says about Ibn IshaqWestern non-Muslim researchers also criticized Ibn IssHaaq:"False ascription was rife among the 8th century scholars and that in any case Ibn Ishaq and contemporaries were drawing on oral traditions." (Cook, M: Muhammad, Oxford 1983. pg. 65)Even a famous anti-islamic scholar criticize the work of Ibn IshaqThe famous Polemist and anti-Islam author Robert Spencer admits in his book The Truth about Muhammad , that"However, Ibn Ishaq's life of Muhammad is so unashamedly hagiographical that its accuracy is questionable." (Spencer, Robert: The Truth about Muhammad, Regnery Publishers, 2006 pg. 25) Despite his negative opinion regarding Ibn Ishaq, out of the 400 footnotes of Robert Spencer's book, 120 footnote refer to fabricated stories from Ibn Ishaq's book. Because Robert Spencer hates Islam so much that he iswilling to present these stories as if they are true, even though he knows that their source is unreliable.