Regarding the QC issue, Hugo explained that the larger the area covered with gloss varnish, the higher the chance that it will develop air bubbles, forcing them to restart from scratch.
I had one intricate design in particular with a full face gloss varnish that made them regret offering the UV printing service. Let's just say that after a few consecutive failures one of the enclosures in my order had a powder coat so thick that it could withstand an artillery shell.
Lesson learned:
it's best to avoid large surfaces covered with gloss varnish. Now, is it the same for matte varnish? Probably not?