Most of the Garlean characters use modified Elezen models and they tend to look a bit 'off' to me - simply enough it must be the mix of Elezen body proportions and a more 'human' head, but I was interested exactly where the difference is, so I took matching screenshots of Nero and my Elezen character to compare them.
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It seems I got the angle a little bit off, judging from the position of their left hands, but it's close enough for comparision. I matched the image sizes by measuring how many pixels there were between the buckle on the scarf and the belt. You can put the two images in a 'gallery' viewer and flick back and forth to compare.
The heads are the same size and at the same height, but Nero's is wider, so it looks like his neck has to slope inwards slightly to match up with the Elezen body - although they do seem to have modified the shirt collar as well, but maybe not enough? Or maybe it's more about the length of the neck.
Possibly their eyes need to remain at a fixed height when they modify the face shape, so they can't adjust the proportions entirely. At any rate, they are at the same height on these two pictures.
Also noticable that Nero's coat is not simply a recolour of the Alchemist's Coat, but has some modified elements - higher coat collar, wider shirt collar, and an extra layer underneath that's probably hiding the join between the head and body. Nero's scarf is also thicker and (unless it's due to the angle) his shoulders are a bit more rounded.
So yeah. It seems there's not a huge difference between them except for the changed width of the head and neck, but it's noticable, particularly when they're moving. Perhaps also because Nero has quite a realistic face, it gives a different impression (a subconscious expectation that the body needs to look correctly-proportioned?) to the more stylised Elezen designs. An "uncanny valley" sort of effect.