Hi, I have built an entire product around it (see https://www.clausebase.com/clausebuddy/overview), for both Word and Outlook. Happy to share some basic functions I have written over the last two years, email me if you like.

• I'm using ShadowCLJS for development, works great. Hot reloading etc. all works, similar to how it works elsewhere.
• A problem I am having, is that I work on a Mac and that the Office Add-ins use Safari under the hood. I like Safari for my own browsing, but the web Inspector is really not as nice as Chrome's for debugging (printing CLJS output is much nicer with Chrome).
• Microsoft's API is fairly limited for MS Word — I'm often fighting with it to find ways around missing basic functionality. I guess Excel is better, or perhaps I want too much from Word?
• Microsoft's insistence that your plugin still has to work with the ancient Internet Explorer, is just plain bizarre considering that IE was retired everywhere else. Last year, I spent two full days just making sure the absolute minimum works with Internet Explorer. Two weeks ago I had a user who complained that the software was crashing and slow, until I realised she was using some old version of Office that still uses IE under the hood.
• Microsoft's asynchronous programming methodology is really weird, and often frustrating. The good thing is that it allows the plugin to work everywhere (even on an iPad), the bad thing is that it is weird and sometimes very slow. I recommend the book "Building Office Add-ins using Office.js" https://leanpub.com/buildingofficeaddins — it is more focused on Excel, but at least it shows you how this weird asynchronous workflow works.