niggling vs piddling

niggling and piddling both are adjectives.

Word	Noun	Adjective	Verb	Adverb
niggling	No	Yes	No	No
piddling	No	Yes	No	No

As adjectives, niggling and piddling are synonyms defined as:

Other synonyms of niggling include fiddling, footling, lilliputian, little, petty, picayune, piffling, trivial.

niggling (adjective)	piddling (adjective)
(informal) small and of little importance	(informal) small and of little importance