There is not much difference between the C# and C++ API. There are a few exceptions, which are documented in the "API Changes and Limitations" section of the documentation. You can find the Unity Integration specific documentation inside your Unity project, under Assets\Wwise\Documentation. But I agree, this doc is somewhat thin on details.
That said, for the 3 features you list:
- Dynamic bussing: I guess you are talking about Auxiliary Sends? That's supported and works through the SetGameObjectSendValues function, like in the C++ API. You can check in the Assets\Wwise\Examples folder, in the files AkEnvironment.cs for an example on how to use that if you want to do your own system.
- External sources: Limited to one source per event (which is 99.999% of the cases).
- Microphone is not supported in Unity integration yet.
Finally, let me mention that we'll be releasing a new version of the Unity plugin quite soon (next few weeks) with better doc and a much easier project setup, amongst other things.