twenty-eight vs xxviii

twenty-eight is a noun but xxviii is not a noun.

Word	Noun	Adjective	Verb	Adverb
twenty-eight	Yes	Yes	No	No
xxviii	No	Yes	No	No

As adjectives, twenty-eight and xxviii are synonyms defined as:

Other synonyms of twenty-eight include 28.

twenty-eight (noun)	xxviii (noun)
the cardinal number that is the sum of twenty-seven and one

twenty-eight (adjective)	xxviii (adjective)
being eight more than twenty	being eight more than twenty