Even FCK occasionally has issues with pasting content from Word. Plus it's slow to load. It has improved over time, though, and is generally a reliable component. Considering the insane constraints rich text editors must operate under, it does an admirable job.
I tried creating a home-grown editor once. It was hard. Making one that's simple, predictable, and end-user friendly is nigh impossible.
I'm working on a jQuery-based one right now (with a matching Drupal module, which would make it light-weight, as Drupal bundles jQuery). Once you understand the nuances of designMode/contentEditable, things get easier, but I know what you mean about how much you need to consider. It might feel like wasted time when CKEditor 3 (smart name change, in my opinion), which looks like the real deal, is released on December 1st.
I tried creating a home-grown editor once. It was hard. Making one that's simple, predictable, and end-user friendly is nigh impossible.