Same boat here on a 2013 Nissan Pathfinder SL. Bluetooth phone connection has been solid since iOS 8 was installed on the phone. USB connectivity for audio integration/playback hasn't worked since iOS 8. 8.1 didn't fix the issue.
Like others, I get functionality through the Pathfinder head unit to navigate the music on the iPhone - playlists/albums/artists/songs/podcasts are all visible and I can navigate to anything I want to. The problem starts as soon as I select a song/playlist to listen to. I click to select the song/playlist simply nothing happens.
I can control from the iPhone itself - selecting a song/playlist from the phone itself begins playing audio through the USB connection. But nothing is displayed on the system screen.
This phone worked perfectly with the Pathfinder before the iOS 8 update.