witches' brew vs potpourri

witches' brew and potpourri both are nouns.

Word	Noun	Adjective	Verb	Adverb
witches' brew	Yes	No	No	No
potpourri	Yes	No	No	No

As nouns, potpourri is a hypernym of witches' brew; that is, potpourri is a word with a broader meaning than witches' brew:

Other hypernyms of witches' brew include assortment, miscellanea, miscellany, mixed bag, mixture, motley, salmagundi, smorgasbord, variety.

witches' brew (noun)	potpourri (noun)
a fearsome mixture	a jar of mixed flower petals and spices used as perfume
	a musical composition consisting of a series of songs or other musical pieces from various sources
	a collection containing a variety of sorts of things