Hagarism did not start the trend, John Wansbrough, Crone and Cook's mentor at SOAS, actually laid the foundations and aired similar views. It is best to see the revisionist trend as taking the spirit of scholarly criticism within broadly-modern historiography to its ultimate end, and countering more traditional (think traditional orientalist) historiographic trends.

The basic evidence for the claims of the revisionist trend lies in discrepancies between the traditional written account of early Islam, of which we are reliant almost exclusively on far later sources, and non-written sources.

One of the main points, utilized by Donner, is the changing religious nature of early Islam. Rubin over multiple books and articles has argued that Umayyad propaganda purposefully stressed a grand narrative of close connection, legitimacy, and similarities between Judaism, Christianity, and Islam. Many of these connections border on the millenarianist/eschatological. This would fit well with the millenarianist trends within Christianity and Judaism at the time. It even seems that such millenarianism had another round under the Abbasids, with many Caliphs appearing to adopt the title Mahdi (most famously al-Mansur), something Humphreys (Islamic History, Mu'awiya ibn Abi Sufyan: From Arabia to Empire) and El-Hibri (Reinterpreting Islamic Historiography) point out. There are also hints that the full title of Caliph, Khalifat Rasul Allah (Successor of the Messenger of Allah), was developed later, stressing the role of the Messenger vis-a-vis the older version which had a more direct-from-Allah implication.

Beyond this, there are more concretely physical examples. Rabbat ("The Meaning of the Dome of the Rock") and Flood (The Great Mosque of Damascus), through architectural analysis, has pointed out that the dominance of Mecca and Medina within early Islamic sources might be incorrect. Combined with arguments by Shoemaker in The Death of a Prophet, The Dome of the Rock appears to be architecturally far closer to the Great Mosque of Mecca than a normal Mosque, while the traditional explanations of the Dome of the Rock either being a symbol of victory over Christianity (Unlikely given the almost-Universalist religious tone of Umayyad propaganda) or it being a replacement for the Ka'bah, then under the control of the rival Caliph al-Zubayr (Probably an anti-Umayyad propaganda stressing the heterical nature of the Umayyads in order to boost later Abbasid claims), both suffer from shortcomings. On top of this, multiple other early Mosques have a qiblah which does not face towards Mecca, but either towards Jerusalem or other unidentified non-Mecca locales. Mecca is also minor in the oldest sources, and an old narrative of the tomb of Muhammad being in Medina, with some hints that the Hajj originally aimed there not to Mecca and the Ka'bah, also diminished the importance of Mecca.

Where the revisionists fall completely flat is in taking these hints of early Islamic difference to the extreme. Borrut, through studying Syriac language sources and comparing them with Arabic ones has pointed out repeated that they offer an alternative non-Muslim view on early Islam -- and in many ways confirm the broad traditional timeline of the founding of Islam. Similarly, there is certainly some early importance to Mecca and Medina, the fact the Zubayrids clung on to Mecca and Medina and seemed to derive legitimacy from there would imply they were not unimportant within the early Islamic community. Just as Hagarism asserts, Islamic sources of early Islam were rewritten in Iraq. Yet as Gutas (Greek Thought, Arabic Culture) argues, this rewriting had a political as well as religious component and cannot be seen as strict propaganda. Fred Donner is considered a bit of a synthesis between the revisionists and more contemporary approaches (I cannot find an exact name for the trend, unlike the revisionists, but they are not traditionalists or strictly anti-revisionist). He accepts that the revisionist trend pointed out valid discrepancies which must be accounted for, but without the extreme position of widespread fabrication and obfuscation of early Islam that the revisionists perceived. Ultimately, the revisionists helped remind us that Islam, like any religion, is never static.