Sikhi is unusual among the traditions in this atlas in having a scripture whose text was fixed by the tradition's own founders, in their own lifetimes, in a script devised for the purpose.
The Guru Granth Sahib was compiled by the fifth Guru and given its final form by the tenth, and the composition of the Gurus is preserved alongside verse by earlier figures from other traditions, Hindu and Muslim, included on the strength of what they said rather than who they were. The scripture is arranged by musical mode, because it is meant to be sung, and its authority is not merely textual: the tenth Guru vested the office of Guru in the volume itself, which is why it is treated as a living authority rather than as a book about one.
That leaves comparatively little of the textual uncertainty that dominates the study of older traditions. What is argued over lies elsewhere: the status and authorship of parts of the Dasam Granth, and the historical value of the janamsakhi literature, the traditional narratives of Guru Nanak's life, which are devotional works composed after the fact and which vary considerably between recensions.
The historical record is also comparatively good. The period of the Gurus falls within Mughal history, and Persian-language sources, court records and later colonial documentation supply an external frame that older traditions in this atlas simply do not have.