First of all, it is not a reliable reference. I noticed a multitude of errors even in my partial reading: a map of Afghanistan shows half the Wakhan corridor belonging to China; a map locating terrorist attacks by suicide bombers fails to show any in Russia; Indonesia is asserted to "have undergone a successful industrial revolution" that has placed it "well ahead of Iran in terms of economic development" (even in PPP terms, Indonesia's per capita GDP is half that of Iran's); and so forth. And proof-reading seems to have been omitted: we have "as in Minangkabau", as if that were a place rather than an ethnic group, misspelled names such as "Khomein" and "Neguib Mahfouz", misplaced commas, and other problems. Even the index is poorly put together: an incidental mention of Bangladesh in an article on Southeast Asia is indexed, for example, but a whole sentence on Bangladesh in the article on India is not.
The Atlas is US-centric and has some startling omissions. The above mentions are all there is about Bangladesh, with the world's fourth largest Muslim population. A two page article on "Iraq 1950-2003" has two maps just for the 2003 invasion, complete with individual unit designations for the invading forces, but the Iran-Iraq war gets just one paragraph and no map — and isn't mentioned at all in the two pages on Iran since 1500! There's no mention of the genocides in Armenia and Bangladesh. There also seems to be some blindness about the Palestinians: "Flashpoint Israel-Palestine" refers to "the flight of thousands of Palestinians" in 1948 (off by two orders of magnitude) and there is no reference to the significance of the Palestinian issue in the broader Arab and Muslim world.
The maps are low on information density, with a few broadly presented features and little detail. Though mostly workable, the cartography is uninspired — compare, for contrast, the superb Historical Atlas of Indonesia. The maps are often poorly connected with the text. Two pages on the Muslim minorities in China, for example, come with a map of Manchu China that focuses on colonial conflicts with the British and French. The photographs included are similarly mixed. Some are informative as well as attractive, but others seem purely decorative: a generic photo of palm trees used to "illustrate" Ibn Battuta's visit to the Maldives, for example.
The Atlas is only very broadly chronological, with more of the feel of an encyclopedia. With the caveat about its omissions, its best feature is probably its breadth: there are articles on Islamic art, Muslim cinema, water resources (with a map showing groundwater flows in Israel and Palestine), the basic foundations of Muslim belief, and more. The Historical Atlas of Islam is clearly not aimed at serious students of history, but intended as a general introduction to Islam for newcomers. For that purpose it may not be a terrible choice despite the flaws I've catalogued.