dreadfulness vs unpleasantness

dreadfulness and unpleasantness both are nouns.

Word	Noun	Adjective	Verb	Adverb
dreadfulness	Yes	No	No	No
unpleasantness	Yes	No	No	No

As nouns, unpleasantness is a hypernym of dreadfulness; that is, unpleasantness is a word with a broader meaning than dreadfulness:

dreadfulness (noun)	unpleasantness (noun)
a quality of extreme unpleasantness	the quality of giving displeasure
	the feeling caused by disagreeable stimuli; one pole of a continuum of states of feeling