mental rejection vs uncertainty

mental rejection and uncertainty both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental rejection	Yes	No	No	No
uncertainty	Yes	No	No	No

As nouns, uncertainty is a hypernym of mental rejection; that is, uncertainty is a word with a broader meaning than mental rejection:

Other hypernyms of mental rejection include doubt, doubtfulness, dubiety, dubiousness, incertitude.

mental rejection (noun)	uncertainty (noun)
doubt about the truth of something	being unsettled or in doubt or dependent on chance
	the state of being unsure of something