If there is one thing that can be said for Interop it is that the interface of the replacement component is, with rare exceptions, complete and consistent with the original COM API. I do not know of any gaps in the Interop for Excel although a few things might need to be called slightly differently and will require loose binding in C# and reference through object in VB.NET. The primary Interop assembly for Office is even slightly optimized for .NET -- particularly if you are working in .NET 4.0. Anyway, migration teams typically use Interop for Office and .NET unless they have made a deliberate decision not to. There are emulation components from companies like Aspose and SyncFusion. For a customer who did not want to install Office on their servers, we developed Office-to-Aspose rules and wrapper classes.