My guess is that because most of the tribal prefixes are pronounced as a full syllable, it'd be a vowel issue if they don't mix well.
Of course, U'odh Nunh messes this up because the U tribe prefix is "oo" so we get a double vowel, but they don't exactly form a diphthong like say K'ih would.
Of course, U'odh Nunh messes this up because the U tribe prefix is "oo" so we get a double vowel, but they don't exactly form a diphthong like say K'ih would.