The Wikipedia article on Taiwanese says "The voiced plosives (/b/ and /ɡ/) become the corresponding fricatives ([β] and [ɣ]) in some phonetic contexts." In what contexts does this happen?
And more generally, what other allophonic rules are there in Taigi, except /t/, /p/, /k/ becoming, respectively, a flap, [b], and [g] intervocalically (which are just my observations)?
It is true that those phenomenons occurs, but for native speakers, those phenomenons do not really make difference in the meaning of words.
My personal experience is that those phenomenons usually occur after some vowels.
I personally think those phenomenons might just be habits of pronunciation.
For example:
a̍p-á [a-ba] box
hâng-gia̍p [hang-ɣiap] occupation, profession
ti-bah [ti-βah] pork
...