I just replaced my PX3 unit with the PX6 and it is so much better in every way. Apps load instantly as you would expect and switching through the various sources from the Sync 3 and the various streaming apps (Spotify, Tidal, and Amazon Music) is working flawlessly. I am not experiencing the issue with using the steering wheel controls to advance songs on my streaming sources that others have reported. When I clicked the forward or reverse buttons on the steering wheel, the song advances to the next or previous song as expected. My PX3 unit also worked in this manner so not sure why some units are not working as expected. As an fyi, I RMA'd my PX3 unit and paid the difference in price to get the PX6 so I am not out the entire price of the upgraded unit.
Lucky! I think I'll buy another and return the current one