Honestly it's not so much the body that I find distasteful as it is the helmet. I think the revised upper chest armor in RoTS looks alright. I don't like it more than his OT chest armor but I get the seamless look they were going for.
The helmet though... It's wrong in so many different ways. The lack of asymmetry isn't an issue in regards to design, it's the various details and dimensions of the helmet.
The helmet/dome looks too big, and the mask looks too small. I think this is in part because the line between the dome itself and the "skirt" of the dome is isn't sharp enough. It isn't that the "skirt" is coming out too far at the bottom, it's that the "skirt" begins too close to where it ends in regards to horizontal distance. I am sorry I'm not explaining it well.
Regarding the collar of Vader's helmet, I always liked the idea of that the collar of Vader's helmet looked somewhat similar to a neck brace because it was a neck brace, owing to him having broken his neck at some point. In RoTJ, when he is being electrocuted, you can see that the model of his skeleton has something attached to his neck; once I noticed this I assumed the people who designed his skeleton had intended to convey that he had suffered damage to his neck. The RoTS helmet's collar, however, looks too similar to a neck brace because it is too long, and there is a sort of "U" curve to the bottom of it that is very reminiscent of actual neck braces.
The flat lenses look bad. Vader's bubble lenses serve to make his mask more unique, and add considerably to his striking appearance in my opinion.
Finally, the completely gloss black mask looks strange. I feel like this is because the various contours and lines of Vader's mask become lost from a visual perspective when the entire mask is the same color and finish. I know that the Vader helmet in the OT only had a alternating pattern of black and gunmetal gray because all black didn't show up well on the cameras they were using, but I think that Vader's appearance is certainly the better for it. Even in person, I prefer the alternating gunmetal and black pattern to completely black. I think this is because this alternating dual color scheme is particularly suited to Vader's mask, because of the many contours and lines that serve to somewhat divide the mask into sections.
I really hope that the Rogue One costume is able to achieve a completely symmetrical look while still getting the dimensions and various details of the helmet right. Symmetry is fine, but it has to be done right.