So it's been discussed a couple times on the forums that some Roku models can run a 1920x1080 roScreen component, such as here. For the most part it seems people say that if the device supports 4k then it supports running a 1920x1080 UI. However, my current gen Roku Stick and Roku Express run the 1920x1080 roScreen just fine, and they are not 4k streamers. So that means the ifDeviceInfo.GetSupportedGraphicsResolutions() method returns false negatives as to whether a 1920x1080 roScreen resolution is supported.
I'd personally be fine with something as simple as a list of models that do and don't support a 1920x1080 roScreen (I've got a feeling that all current and future gen models will support it).