Some non-Muslim scholars reject all hadith as fabrications, which colors their views. Others, like Wilferd Madelung, accept the hadith literature. A few of them, like Lammens, hold a negative view of Ali. Madelung criticizes this school of thought and, like many other non-Muslim Islamic scholars, praised Ali.
Some other Islamic scholars do not accept narrations collected in later periods, and only study the early collections of narrations. This leads them to regard certain reported events as inauthentic or irrelevant.
Among events that these scholars reject on the grounds that they are not included in what they call "early sources" (meaning, essentially, the Sirat Rasul Allah of Muhammad ibn Ishaq) include:
"work with the narrative sources, both those that have been available to historians for a long time and others that have been published recently, made it plain that their wholesale rejection as late fiction is unjustified and that with [not without] a judicious use of them a much more reliable and accurate portrait of the period can be drawn than has so far been realized."
The zeal and virtue of Ali were never outstripped by any recent proselyte. He united the qualifications of a poet, a soldier, and a saint; his wisdom still breathes in a collection of moral and religious sayings; and every antagonist, in the combats of the tongue or of the sword, was subdued by his eloquence and valour. From the first hour of his mission to the last rites of his funeral, the apostle was never forsaken by a generous friend, whom he delighted to name his brother, his vicegerent, and the faithful Aaron of a second Moses.
Endowed with a clear intellect, warm in affection, and confiding in friendship, he was from the boyhood devoted heart and soul to the Prophet. Simple, quiet, and unambitious, when in after days he obtained the rule of half of the Moslem world, it was rather thrust upon him than sought 
In my view, ʿAlī was the first Arab to have contact with and converse with the universal soul. He died a martyr of his greatness, he died while prayer was between his two lips. The Arabs did not realise his value until appeared among their Persian neighbors some who knew the difference between gems and gravels.
He had a contempt of the world, its glory and pomp, he feared God much, gave many alms, was just in all his actions, humble and affable; of an exceeding quick wit and of an ingenuity that was not common, he was exceedingly learned, not in those sciences that terminate in speculations but those which extend to practice 
He was of the noblest branch of the noble race of Koreish. He possessed the three qualities most prized by Arabs: courage, eloquence, and munificence. His intrepid spirit had gained him from the prophet the appellation of The Lion of God, specimens of his eloquence remain in some verses and sayings preserved among the Arabs; and his munificence was manifested in sharing among others, every Friday, what remained in the treasury. Of his magnanimity, we have given repeated instances; his noble scorn of everything false and mean, and the absence in his conduct of everything like selfish intrigue 
He was one of the last and worthiest of the primitive Moslems, who imbibed his religious enthusiasm from companionship with the Prophet himself, and followed to the last the simplicity of his example. He is honourably spoken of as the first Caliph who accorded some protection to Belles-Lettres. He indulged in the poetic vein himself, and many of his maxims and proverbs are preserved, and have been translated in various languages. His signet bore this inscription: 'The kingdom belongs to God'. One of his sayings shows the little value he set upon the transitory glories of this world, 'Life is but the shadow of a cloud - the dream of a sleeper' 
Valiant in battle, wise in counsel, eloquent in speech, true to his friends, magnanimous to his foes, he became both the paragon of Muslim nobility and chivalry (futuwah) and the Solomon of Arabic tradition, around whose name poems, proverbs, sermonettes and anecdotes innumerable have clustered.” 
As the chief of the family of Hashem and as the cousin and son-in-law of him whom the Arabians respected …, it is apparently wonderful that Ali was not raised to the Caliphate immediately on the death of Mohammad. To the advantages of his birth and marriage was added the friendship of the Prophet. The son of Abu Talib was one of the first converts to Islamism and Mohammad’s favourite appellation of his was the Aaron of a second Moses. His talents as an orator, and his intrepidity as a warrior, were grateful to a nation in whose judgement courage was virtue and eloquence was wisdom." 
In the Divine Comedy, Dante describes in Inferno XXVIII that both Ali and Muhammad were in the 8th (and second harshest) Circle of Hell, as sowers of dissent:
see how mangled is Mohammed!
Ahead of me proceeds Alì, in tears, his face split open from his chin to forelock.
'And all the others whom you see
sowed scandal and schism while they lived,
and that is why they here are hacked asunder.