"if "pure, unadulterated spoken vernacular Taiwanese" were written exclusively in Chinese characters, with minimal use of Mandarin phrases, over 25% of morphemes would have no character, about 25% would have arbitrarily selected (yet more or less conventionally accepted) characters that are homophones or near-homophones, 10% would be written using characters exclusive to Hokkien, and 40% would be written with characters that have the correct sound and meaning. However, in more colloquial styles of Taiwanese Hokkien, the proportion of morphemes written with conventionally accepted characters would drop even lower than 40%."
Calling languages like Hokkien “a slightly different dialect” is really underselling it. These are totally different languages, as divergent as German is from Portuguese.
There are Hokkien-speaking people in Hong Kong and China's Guangdong province. They write Hokkien in Chinese characters mixed with pinyin for these without a corresponding character.