I use my iPhone 4 hooked up to an Alpine headunit in my vehicle. With apps like Pandora all the Track info and album art is passed through and displays on the head unit correctly. Spotify will display them but most of the time there will be incorrect info, the album art won't load, or the time of the track will be incorrect. Also the headunit buttons will only work to skip tracks, pause etc some of the time.
I know it's not an issue with my H/U or phone as all other music apps work fine.