Stress is determined based on the syllables, not the vowels, so the
diphthongs need to count as a single syllable. If there is a diphthong
at the end of a word, the penultimate syllable can still be unstressed
if it is light. (The name of Arvedui, last king of Arthedain, is one
example of this that I could think of, but I’m sure it’s not the only
one.)