The platter and mat are different. The acrylic mat is the same thickness as the cork just made from acrylic. For the uturn, you need a mat to keep the VTA right. I think the projects have an acrylic platter that doesn't need a mat because it's thicker? Someone else want to chime in on this?
Typically no adjustment needed, but check your turntable's manual to be sure. For my Pro-Ject, for instance, the acrylic platter is as thick as the standard platter plus mat as it's a direct swap so no adjustment is needed. As for dust, I usually just brush the top off with my carbon brush before putting a record on. Never had any issues with scratching, etc
Seems like it's a design issue with the UTurn. They didn't design the acrylic platter to be tall enough without a mat and you can't adjust tonearm height on UTurns so a mat is still required making the benefits of an acrylic platter purely aesthetic