It isn't. It falls slightly apart in the `s` column, and completely in the `t` column which contains both "chi" and "tsu". It also breaks for godan words that end in "u" which become "wa" in the negative form.
Mu, bu and nu also all obey the -nda transformation due to phonetics, and not due to how "if we just shuffle the letters around and presto! Nomu becomes nonda".
Japanese already has plenty of its own reading inconsistencies, so adding another layer on top isn't going to help you.
Finally, there's going to be so much kana in your every day life that learning conjugation in romaji is guaranteed to cripple your reading, because instead of recognizing kana (e.g. you see a billboard that says お茶を飲んだ方がいい！ as you frantically try to back-translate everything into romaji, but also removing excess w's and converting nda's as you go) you've spent the first n hours on trying to "hack" the language instead of just learning it.
