underbelly vs weak spot

underbelly and weak spot both are nouns.

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

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

Other hypernyms of underbelly include soft spot, weak part.

underbelly (noun)	weak spot (noun)
the quality of being weak or unprotected	a place of especial vulnerability
the soft belly or underside of an animal's body
lower side