piece of cake vs pushover

piece of cake and pushover both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of cake	Yes	No	No	No
pushover	Yes	No	No	No

As nouns, piece of cake and pushover are synonyms defined as:

Other synonyms of piece of cake include breeze, child's play, cinch, duck soup, picnic, snap, walkover.

piece of cake (noun)	pushover (noun)
any undertaking that is easy to do	any undertaking that is easy to do
	someone who is easily taken advantage of