I suspect the format of this book is the biggest factor that accounts for its popularity among homeschoolers. The text is broken up into manageable chunks rather than overwhelming chapters. Frequent illustrations add visual variety. Topically, it sticks with the Protestant side of Christianity aside from a brief look at Islam (beginnings of the religion and the Crusades).
Also, you can read a section from the middle of the book that pertains to another topic in which you are interested without having to read the entire book. That means it might work well as part of a unit study.
It is written at adult level, so it will not serve as a student book. It might be read aloud with fifth or sixth graders, but I suspect that most of them will be overwhelmed if it is not pre-read and summarized by parents. When your children reach junior high and high school levels, they can read it themselves.
Sketches covers much material that is skipped or glossed over even in Christian history texts. However, because it focuses on the Protestant Church (particularly from a Reformed perspective), early church history is not adequately covered and an anti-Catholic bias is very evident. In addition, its scope is limited geographically to Britain, Europe, and America and it covers church history only up through the nineteenth century.