Well, different accents do have different phonetic allophones for phonemic diphthongs.
I’d SKIM the article at the link.
Remember that this is also taking in account the rising diphthongs not just the falling ones.
Plus the triphthongs, which most of us never have to deal with in our own languages.
Except for that we don’t count as triphthongs things like those in English because of rhyming.
Of course an Iberian word with a stressed triphthong like averiguáis, averigüéis (ind+subj vosotros forms of "verify") would likely rhyme in their poetry differently than in ours.
It would probably only need to rhyme with /a/ and /e/ respectively, not worrying so much about the glides to either side. I’d have to hunt to see if they rhyme /ai/ with /a/, /ei/ with /e/.
But in English we can use rhyme to show the rising diphthongs are not thought of as different vowels than a version without the on-glide.
So quick can rhyme with sick. We don’t care about the /w/ on-gline.
I’m less certain of cute and shoot rhyming, but quite possibly.
In which case, the /ju/ diphthong doesn’t count for rhyming, just the /u/ part does.
Portuguese preterite averiguei is spelled averigüé in Spanish, and there is no /j/ off-glide there.
Portuguese lost their diaereses because they thought you should be able to know whether the u is silent to make the g hard or whether it should be a /w/ sound. Spanish thinks that's dumb to make people guess so always writes things consistently and predictably based on sound alone. Hence lingüística, which is how the Portuguese spelled it until recently as well.
If you saw the word guisa you would not know whether that were /gwisa/ or /gisa/ if it were Portuguese, but Spanish guarantees that it must be only /gisa/ because /gwisa/ would have to be written güisa.
And yes, guisa means the same obvious thing in all three of Italian and Spanish and Portuguese.
The Italians don't use it much though.