Most new units these days have the bluetooth function (and microphone in a lot of cases) integrated.
Those with an external mic will give you better audio quality.
You can easily get ISO to dual fakra radio antenna adapters. A lot of Chinese units have none or very poor AM integration.
Using the VW RGBS reverse cameras is difficult, in most cases its probably easier to just install an aftermarket camera. Though if you have a highline (with bendy guidelines) camera this isn't what you want to do.
There are units which integrate the factory OPS sensors properly.
The only MFD functions you may lose are the navigation prompts and media info, all other functions are not dependent on the RNS.

I am holding out hope that there will be a way to get the PQ MIB units working in our older cars. There are videos of them in operation with a lot of original functions working (camera with guidelines, OPS, radio, MFD integration etc as well as the new functions like Mirrorlink, Android Auto etc. I am actively looking into this though it also requires that some factory parts be upgraded to support it such as Canbus gateway, camera controller, PDC controller, amplifier etc. Not sure if I want to do it more from the perspective of a challenge (I suspect this is the case) or if I just really want to stay with factory components...