Another major issue was that the first implementation most people saw of it was the email side, and it could be a truly clunky and unpleasant email client. This soured opinions before people delved into the document management and programmability features that email handling were just one use of.

From a technical point of view one of the bonuses I saw was that it used PKI throughout for encryption and such, which very little other software did. Though this was also clunky at times especially for non-technical users (has anyone ever made the use of PKI a smooth process for those who don't care to know the details?). Proper ACL management too rather than more simplistic permissions, but again this could be very clunky.

Though I'm not sure why we are talking entirely in the past tense, while Domino & Notes are not widely used anymore, they are still out there and developed (under the name HLC Notes) with the last release (adding LLM based “AI” features, of course…) was Jun last year and a bugfix update a few months later. My experience with Domino/Notes was in the 00s and early 10s when I was the accidental admin (the only guy who really understood it left) of a mail and document server based on it, hopefully the clunkiness complaints at least have been addressed since then.