It may be just me, but I hear a strange similarity between sounds that are clearly different, viz.
All four diphthongs go from half-open in the direction of a close sound. That is, /o~i/ could be expected to start half-close, because the monophthong /o~/ is also half-close, but in fact it usually starts much opener than that.
The Dutch diphthong is front (but not quite) and rounded, the Portuguese diphthongs /3i/ and /3~i/ start central and unrounded. The acoustic effect of rounding a front sound, and centralising a front unrounded sound is different, but often seems similar, to my ears at least.
(Also read and listen to this.)
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Linguas de correspondentia:
nl, ia, en, de, pt