Avira, many Chasidishe Sefarim are indeed a form of Mussar or Avoda. Chabbad Sefarim are not. They are Toras Hanistar as explained by their Rebbes. That settles two issues. It gives clear explanations to the point being discussed and it is Limud.
Even other Chasidishe Sefarim are obviously Divrei Torah, as is Pirkei Avos. The only kind of Sefer I would question is something that focuses on Eitzos of how to work on a Midda. This is usually the author’s life-advice, and could just as well have come from Benjamin Franklin. (Include all disclaimers)