Losing one's way
Making sure that crucial nodes are visited is more difficult if the hypertext has no clear tree structure, so there are fewer structural clues to guide the reader. This may come about because added cross links have obscured an originally intended tree, or because the text was deliberately created without any visible tree.
The one or more spanning trees that will be present in any connected graph are not the same as the navigational tree structure often used to organize the reader's impression and experience. Issues of network design and efficiency are not the same as issues of cognitive mapping and understanding.