Some simple calculations about rhyming in English.
This uses nltk to find all the English words that, as per the CMUDict, have no rhymes.
It then uses wikiwords to find, per frequencies in the 2012 edition of wikipedia, the most common unrhymeable words.
The output as of 2.15.17:
29.23% of English words are unrhymeable.
The 10 most common unrhymeable words are: also, january, april, category, comments, album, several, subsequent, district, modify.
761 words end in the stem "EY1 SH AH0 N".
(re: the inclusion of 'modify': it looks like CMUDict doesn't have the most normal pronunciation of 'codify')