labour of love vs undertaking

labour of love and undertaking both are nouns.

Word	Noun	Adjective	Verb	Adverb
labour of love	Yes	No	No	No
undertaking	Yes	No	No	No

As nouns, undertaking is a hypernym of labour of love; that is, undertaking is a word with a broader meaning than labour of love:

labour of love: productive work performed voluntarily without material reward or compensation
undertaking: any piece of work that is undertaken or attempted

Other hypernyms of labour of love include labor, labour, project, task.

labour of love (noun)	undertaking (noun)
productive work performed voluntarily without material reward or compensation	any piece of work that is undertaken or attempted
	the trade of a funeral director