Well there are a lot of threads about that here.
Here are some from the top of my head: often failing the firmware update, which apparently causes the bt module to be bricked. SD-card features suck tbh (quite often failing to initialize it - needs a reboot; the ID-tag playback problem I made a thread about; makes duplicate artists in the ID-tag category, although the name is exactly the same on my PC and the unit). Doesn't show artist-track info when playing audio CD-s (yes, the CD-s have that information and they work on my old 1 DIN unit).