lang syne vs long ago

lang syne is a noun but long ago is not a noun.

Word	Noun	Adjective	Verb	Adverb
lang syne	Yes	No	No	Yes
long ago	No	No	No	Yes

As adverbs, lang syne and long ago are synonyms defined as:

Other synonyms of lang syne include langsyne, long since.

lang syne (noun)	long ago (noun)
past times remembered with nostalgia

lang syne (adverb)	long ago (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past