A disadvantage of using a sentence outline instead of a topic outline is that a sentence outline is often time-consuming to develop.
A topic outline organizes your ideas hierarchically from main to sub-points, in the sequence you want, and shows what you will talk about. It identifies all the little mini-topics that your paper will comprise, and shows how they relate.
A sentence outline does the same and whay is more, it shows exactly what you will say about each mini-topic. Each sentence, instead of simply identifying a mini-topic, is akin to a mini-thesis statement addressing a mini-topic. It conveys the specific and whole idea that that section of the paper will cover in order to prove the overall thesis.