stumbling block vs obstacle

stumbling block and obstacle both are nouns.

Word	Noun	Adjective	Verb	Adverb
stumbling block	Yes	No	No	No
obstacle	Yes	No	No	No

As nouns, obstacle is a hypernym of stumbling block; that is, obstacle is a word with a broader meaning than stumbling block:

stumbling block: any obstacle or impediment
obstacle: an obstruction that stands in the way (and must be removed or surmounted or circumvented)

stumbling block (noun)	obstacle (noun)
any obstacle or impediment	an obstruction that stands in the way (and must be removed or surmounted or circumvented)
	something immaterial that stands in the way and must be circumvented or surmounted