Multiple threads on this site, if you do a little search. Basically, inconsistent performance. Random re-loads of USB drive contents; random “file not recognized”-type errors when playing; lack of shuffle mode that actually shuffles songs; sometimes meta-data is displayed (artist/album artwork) and sometimes not; that sort of thing. I have also had GPS issues where the car can’t figure out where it is sometimes, which throws off scheduled charging and Homelink functionality, and the phone support folks tend to blame whatever is plugged in to the USB ports when that happens. Eventually I stopped using my USB stick due to the GPS problems, which does seem to have helped, so now I just use the streaming audio in the car.
I haven’t heard much about USB functionality in the 3, so maybe they have these issues fixed and the fixes just haven’t worked back into older cars yet. I also haven’t used my USB stick for a few months, so it’s possible a recent update fixed these and other issues and I just don’t know about it although my assumption is that any fixes would soon be broadcast all over this site as it seems many people have the same issues.