Hadith collections serve different research purposes. Their organization does not by itself determine whether their narrations are sound or weak; it determines how a reader locates reports.
Musnad: reports grouped under the companion from whom they are narrated.
Muʿjam: a collection organized alphabetically by names or other designated headings, often teachers.
Abwāb: reports organized by legal or topical chapters, such as purification, prayer or commerce.
A thematic work may repeat one report in several chapters for distinct legal implications, while a musnad generally directs the researcher first to its companion narrator.