If your ideas seem hazy, you might not feel ready for the detailed development a sentence outline requires. In this case, a topic outline might prove advantageous because you can organize thoughts without necessarily articulating precise points. For example, one Roman numeral in a topic outline might say, "Define the problem." You can't get away with such a vague phrase in a sentence outline. Instead, you would state the specific problem in a complete sentence -- a disadvantage if you're not ready.
Because sentence outlines feel almost as detailed as first drafts, you might fall prey to lack of confidence and your nitpicking "inner editor." Picking at details puts you at a disadvantage when it comes to big-picture questions such as organization. By contrast, topic outlines let you map ideas without your inner editor chiming in because you don't have to worry about grammar or flow. On the other hand, sentence outlines can boost your confidence while writing the first draft because each section translates to a fairly complete paragraph.
Writing a sentence outline demands time, a disadvantage if you need to meet a tight deadline or tend to wait to the last minute. On the other hand, sentence outlines can speed up the writing of your first draft, so you might find them well worth the time investment. It depends on your process and particular needs.
Sentence outlines fill up the page with text, and individual lines do not stand out the way they do in a topic outline. This may make it challenging to trace the structure of your argument or see where it derails. By contrast, the bare-bones simplicity of topic outlines allows you to identify off-topic points or illogical ordering. The structure of your argument leaps out at you because each line only contains one key idea or point instead of a complete sentence.
Writing complete thoughts for a sentence outline can make ideas seem set in stone, frustrating any attempts to revise later on. You might feel too invested in the ideas because of the labor involved in crafting complete sentences, or you might fail to notice faulty reasoning because ideas seem fully developed. On the flip side, a topic outline might feel too fluid during the drafting stage, leaving you without a clear road map.