underbelly vs soft spot

underbelly and soft spot both are nouns.

Word	Noun	Adjective	Verb	Adverb
underbelly	Yes	No	No	No
soft spot	Yes	No	No	No

As nouns, soft spot is a hypernym of underbelly; that is, soft spot is a word with a broader meaning than underbelly:

Other hypernyms of underbelly include weak part, weak spot.

underbelly (noun)	soft spot (noun)
the quality of being weak or unprotected	a place of especial vulnerability
the soft belly or underside of an animal's body	any membranous gap between the bones of the cranium in an infant or fetus
lower side	a sentimental affection