Aside from the dipthong *issue (which I believe is a "subjective" one), the throat and facial muscle tensions needed to create certain vowel sounds is an important reason for vocalists to avoid their use, as these tensions will easily interfere with the resonance the voice is allowed when relaxed... not to mention (again) the enunciation and euphony.

BYB ...but there is *nothing like a choreographed group of reptiles.