This is just from memory, the impression I got from reading about it in some research, it's a very close relative of C. reticula (mandarin) but more like a wild mandarin cousin. The flavor is probably a bit more bland, less sweet, and definitely inferior to a normal mandarin, but not distasteful.
C. tachibana has apparently been used in the past to breed several other native Japanese citrus varieties. It is the only citrus species believed to have been native to the islands of Japan.
It has moderate fairly good tolerance and apparently is quite accustomed to the growing conditions in Japan. (Which probably means its native climate is zone 9 but it can also survive zone 8, possibly with a bit of difficulty further north.
It's probably adapted to a somewhat cooler maritime climate than typical citrus.
You can take that for whatever it's worth. (Just some rough information in case you cannot find a better accurate source)