I used C# to write an Excel toolbar. The big problem I had with it is that there were features they had only implemented in Visual Basic/C++. It appeared that C# was incomplete in it's implementation of MS API's
I've done the same. The problem isn't C#'s access to the APIs, the problem is the APIs are incomplete, especially as MS moved to the SDI interface since Office 2010. There are somethings you just can't do in an Add-In that are obviously possible due to built-in features. Things like have a text-box in a toolbar show the proper text in inactive windows.