From Reconsidering the Nganasan vowel system (Fejes 2021):
One argument for the vowel sequence analysis is that Nganasan long vowels and diphthongs are twice as long as a single vowel (Helimski 1998: 485). This must sound strange for a Uralist, since Finnish long vowels and diphthongs are also twice as long or even longer than short vowels (Hakulinen 1941: 227); however, they are not analyzed as vowel sequences in grammars.
This doesn't make sense to me since I was under the impression that Finnish long vowels and diphthongs are analysed as vowel sequences in grammars (e.g. Karlsson 1969). Plus it's hard for me to think of any reasonable arguments for why they shouldn't be treated as vowel sequences, especially as there are even examples of Finnish diphthongs being pronounced with hiatus in songs.
Are there actually arguments for treating Finnish diphthongs and long vowels as single segments, and if so what are they?
Anecdotally I don't hear them as single segments as a Finnish speaker, but I'm not sure how universal this is.
