The listing issue is interesting. I looked at a bunch of covers and there seems to be a mix of listed and unlisted. The normal commodity plates, made by major manufacturers bear a listing mark on the package but there are plenty of "designer" plates that do not. As an inspector, if it looks OK (sturdy. reasonable sealing and no combustible material facing the inside of the box) I would not give it a second thought. I actually had to look hard to see a "U/L stamp" on a Leviton but it is there.
Products are 'supposed' to be evaluated/tested by a NRTL, such as UL, or others that are approved here in the USA
Do all 'products' have 'labels' to show the NRTL? Short answer is NO.
Cover plates for switches, receptacles, etc., are IMHO a 60/40 mix. Have I had any issues? As Greg said, if it looks 'good', it's not an issue. In order to see any label. the cover would have to be removed, as the NRTL label would be on the backside. (Who has time for that?)
The idea of making that product is quite fresh, and to be honest I am a bit worry, you know...if something goes wrong, I could end up in a jail if I dont have that US stamp which claim the product is safe....
This is not something you go to jail for but you might end up in civil court if your product was seen as causing someone harm. Unfortunately, in the US, that is always a concern, no matter what you are doing. We just have too many lawyers.