The (Beijing) Chinese vowel in 'feng' only occurs before -ng. It's nothing like the -u of 'wu', but resembles the sound of old-fashioned RP -u- in 'hung'. This is not how 'hung' is pronounced in modern English. So there is no obvious way of transcribing it. Chinese clear -e doesn't occur by itself (i.e. without a following consonant) so it's going spare and can be used for several of the obscure vowels. For example, it's also used for a sound that only occurs before -n, and which is like nothing in English. Wade-Giles used it in several such ways and Pinyin uses it similarly but not exactly the same.

'shui' could be written 'shuei' (i.e. "shway"), but because the actual -ui sound never occurs they seem to have used it for -uei, saving themselves one letter.