So we have a weapon transmutation device thing that changes the appearence of a weapon by consuming another weapon in the process. But where is the unit transmutation device? There are so many good units out there that look great and should outright be accessories, but they dont have the stats to be even worthwhile equiping.

where are they?

Unfortunately doesn't exist here or in Japan, seems to be an ignored thing for some reason.

Viva la gizmod city!

Yea, doesn't exist on JP either, and like over 90% of JP players have their units on stealth anyways. I believe units are also invisible in NGS so don't really expect to see unit transmutation.