Useless is an exaggeration. But having a card that can do 1-4 GB/sec and a card that can do ~300 MB/sec (and I think that is read speed) does introduce issues when you are pushing the limits of the camera. For example, 12 fps x 60 MB (assuming) files = 720 MB/sec. With the CFExpress card, you do not need to buffer that. With the UHS II card, you can not write directly to the card, thus it will need to be buffered. So, this is like having a Ferrari tethered to a Honda. Nothing is wrong with the Honda, other than to get the full performance out of the Ferrari you will need to cut the tether. In doing so, now you are shooting to a single card slot. For most applications this is not an issue and the low cost of the SD card and the fact that SD card readers are common is appreciated.
So, what did we just learn? The R5 will have its burst rate limited by buffer capacity if you want to shoot to both card slots. So starts the ways that we will see the R5 limited compared to the 1DX III. And this is not me whining. This was expected, it is a $3k camera compared to a $6,500 camera.