Finally had a chance to play around, and yup, Limited RGB when playing HDR content is totally borked. Full looks good, though.
On SDR content, Limited and Full should look similar as long as your display has the corresponding setting. I had been using Limited, but I've just decided to switch to Full since it looks just as good and doesn't seem to be messed up for HDR content. Of course, we'll need to see more games with HDR on PS4 to really see what the deal is.
Could the fact that Limited RGB has issues have something to do with the Deep Color setting on PS4? It needs to be on for HDR to work at all.