TinyMCE is not viable if users will be pasting content from Word documents, and generally does a terrible job of dealing with pasted styles...nearly impossible to override, so you end up pasting into Notepad, etc. etc. At my job, we're going to migrate our pilot Drupal sites from TinyMCe to FCKEditor due to this reason alone: To our users (content editors/admins who were stuck in Contribute before), this makes the Drupal software as a whole look bad. Even with the "Word Paste" plugin installed for TinyMCE, users have to click a special "Word Paste" button. This is silly, in FCKEditor you just paste normally.
One of my least computer-savvy clients for whom I built a Drupal site in the spring had absolutely no trouble using FCKEditor, while even I have a hard time wrangling some content in TinyMCE.
In short, TinyMCE's time has come and gone. It's still a decent RTE, but FCK has definitely surpassed it.
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.
What I'd suggest for pasting Word content, regardless of editor, is to add a button that first puts the Word content through the server-side wvHtml tool, which extracts clean HTML from Word's awful formatting. I know TinyMCE has a plugin API for adding a new button for this to the editing options, and I'm sure the others do too.
One of my least computer-savvy clients for whom I built a Drupal site in the spring had absolutely no trouble using FCKEditor, while even I have a hard time wrangling some content in TinyMCE.
In short, TinyMCE's time has come and gone. It's still a decent RTE, but FCK has definitely surpassed it.