How does unoconv differ from using libreoffice --headless --convert-to?

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unoconv has 2 modes: either you use a long-running soffice process, in which case you get fast conversion or you start a new soffice process for each document, which increases security. You need to choose between the two. soffice --convert-to is similar to the "secure" unoconv mode.

BTW, Online also allows document-conversion and tries to have both speed and security at the same time, you can read about that here.