I collect polymer banknotes religiously, I haven't had any graded by the companies, but I know a bit about how the notes behave.
Most polymer banknotes have intaglio printing, and should behave the same as paper if pressed as the embossing would not be as strong. Polymer notes might survive being put in a wallet without affecting it's condition, but it will show a fold or counting mark just as easily as paper notes. One more thing you see is if the notes were banded together using elastic bands, you will see wavy edges on the notes as well. Finally, polymer notes also seem to be prone to having their design wear off, a heavy crease might leave a white line on the note, and I've seen notes so heavily creased that the lines were clear. How the substrate is made is quite interesting, it's basically a clear film that's coated with matte white paint multiple times, then the lithograph printing is done over the white painted areas.