I think the new design is a fine twist on the original v4 spicy. The cut of the outfit really isn't different from the old design. v4 Una had the same exposed belly, a thigh-high tight, a fishnet stocking, a miniskirt, and although she had sleeves, the sleeves had holes. Everything, including the boots, had the same "shape", with the exception of her new peacoat, which I think is a great addition. I personally think the new design gives the impression of being more modest if that is the concern. I think there is just less ornamentation on the design to distract you from the small details of the clothing-- the biggest change is definitely that these are just a touch simpler overall. Might be the shape language.
We have seen dozens of variations on Una at this point which is why I think for me personally the new designs aren't necessarily 'stand-out' in my eyes. I think they're plenty cute and I think the contrasting saturated colors is fun, but I wouldn't describe them as an "evolution" of Una-- more of a different coat of paint.
The XSYN plugin looks like it'll work! But what is actually happening? Do they have two tracks, one with the desired Spicy sound and one with the desired Sweet? And then the automated plugin is interpolating between them?
Maybe this is off, but I think this VB sounds the most "vocaloid" of all of the v6 AI banks so far, but maybe it's just the talent of the voice actor being able to replicate the voice so well. I wonder what niche these VBs will find. I worry that the exclusion of some features from v6 AI voicebanks will limit its adoption as v4 Una really hasn't felt "out of date"... at least in my opinion. But I still feel optimistic.