bit by bit vs piecemeal

bit by bit is not an adjective while piecemeal is an adjective.

Word	Noun	Adjective	Verb	Adverb
bit by bit	No	No	No	Yes
piecemeal	No	Yes	No	Yes

As adverbs, bit by bit and piecemeal are synonyms defined as:

Other synonyms of bit by bit include in stages, little by little.

bit by bit (adjective)	piecemeal (adjective)
	one thing at a time

bit by bit (adverb)	piecemeal (adverb)
in a gradual manner	a little bit at a time
a little bit at a time