What is this they say about Notes C API being difficult ?

Let there be no doubt. The Notes C API can be tricky mainly because of it's complex input- and output structures. By gaining a good understanding of the Notes C API you gain unparalleled understanding of the Notes database construction as well. The effort you invest in understanding the C API will reward you in all other aspects of Notes development. As you'll see in the next subjects, both Notes C++ API, Java API and LotusScript closely resembles the core construction of Notes. Understanding how it works on a low level, never hurts!

However, you certainly don't need a deep understanding of the Notes C API to get your API applications rolling! As the following subjects will show you, you can come pretty far without knowing the Notes C API at all.