Curiously enough, Hepburn romanization fixes some ambiguities in Japanese (Japanese written in kana alone) while introducing others.
The ō in Hepburn could correspond to おう or おお or オー. That's an ambiguity.
Where does Hepburn disambiguate?
In Japanese, an E column kana followed by I sometimes makes a long E, like in 先生 (sen + sei -> sensē). The "SEI" is one unit. But in other situations it does not, like in a compound word ending in the E kana, where the second word starts with I. For instance 酒色 (sake + iro -> sakeiro, not sakēro).
Hepburn distinguishes these; the hiragana spelling does not!
This is one of the issues that makes it very hard to read Japanese that is written with hiragana only, rather than kanji. No word breaks and not knowing whether せい is supposed to be sē or sei.
There are curiosities like karaage which is "kara" (crust) + "age" (fried thing). A lot of the time it is pronounced as karāge, because of the way RA and A come together. Other times you hear a kind of flutter in it which articulates two A's.
I have no idea which romanization to use. Flip a coin?
The ō in Hepburn could correspond to おう or おお or オー. That's an ambiguity.
What's the issue here? They all sound exactly the same, although おお seems unusual. The choice of kana kinda depends on the what you're writing.What's interesting is that they address this problem where the latin alphabet introduces the ambiguity (Is genin げんいん or げにん? Hepburn goes with gen'in for the former to avoid ambiguity), so they could have extended that to sake'iro and applied the same strategy when the ambiguity comes from kana itself.
> In Japanese, an E column kana followed by I sometimes makes a long E, like in 先生 (sen + sei -> sensē).
While it is sometimes difficult to discern the combined E and I sound, especially for non-native speakers, the word 先生 (sensei) is technically pronounced "sensei" and should be spelled that way to distinguish it from words with long E sounds, such as ええ (ee) and お姉さん (oneesan). Similarly, the OU in 東京 (toukyou) and the OO in 大きな (ookina) are different and should be spelled differently. I hope this helps.
EDIT: Added a comma.
> There are curiosities like karaage which is "kara" (crust) + "age" (fried thing).
Slightly off-topic, but “karaage” (kara + age) isn’t “crust + frying.”
The kara comes from a country name and refers to a style of cooking — it’s a “country-name + cooking method” compound.
this is the commonly accepted explanation, though whether it’s strictly historical or a later interpretation is still debated.
If you fry something without coating it, that’s usually called “su” (plain) + “age” (frying) instead.