I contacted support over the course of several weeks, and went through all the bells and whistles to try to fix this thing. Originally on two machines (had plenty of space, RAM, everything) the download would get to exactly 19.21GB and stop. I tried un-installing the launcher, deleting the download and re-downloading, nothing worked.

Yes, I had plenty of space. I even tried downloading a different AC game (I believe it was AC: III) and it was 40GB and downloaded the whole thing no problem.

So, all the possible issues: ports on PC or firewall, DNS, hard drive speed, connectivity: None of those make any sense, both b/c the download would start - AND I could download other games completely that were much bigger.

I finally tried it on another machine that I had to re-install the Ubisoft Client fresh. And it again stopped at 19.21GB of the ~51GB download.

I was like.. wait a second. It HAS to be something between Ubisoft and my house. Which left my ISP as the culprit.

When I moved in about 5 years ago, I used to have to troubleshoot websites from my house. I distinctly recall a time when I was editing one on the fly, and I couldn't see the changes on my browser at home - though, during the course of troubleshooting, I could actually see the changes when I browsed from my phone. Started to drive me crazy, until I realized, Xfinity (Comcast) must have been using some type of data cacher to save on bandwidth. (Download an entire website etc once to their data cache, and they save at least half the cost of repeated downloads by their customers...)

So, I installed a VPN, which forced the download OUTSIDE or AROUND whatever the heck it is that Xfinity was doing (like potentially caching Ubisoft's entire website), and it finally completed the download.

What a pain in the butt.

But, I wanted to post this here in case it helps anyone else in the same situation.