One thing I haven't seen discussed in leaks regarding BC is CD support/playback. Unless there's something being done via software, there's no way PS1 discs could be read. If you recall, PS4 never had CD support (to the chagrin of many prior to launch). Also, technically haven't seen DVD support brought up either, which would be required for PS2 games.
Would there be any actual reason it couldn't play DVDs?
I don't think people would be -that- upset if it supported PS1 BC digitally only.