The app store / Office admin stuff. When I was dealing with this stuff 2 years ago, it was insanely difficult to deploy a web add-in internally (something involving the "Microsoft 365 admin center"), let alone deploy it to customers (gotta get approval to deploy it in the special Office app store, and hope+pray that your customers' IT departments don't block that).
Compare to VSTO, where IIRC installation just involves editing a registry value to let Office know where your add-in can be found on disk.
Compare to VSTO, where IIRC installation just involves editing a registry value to let Office know where your add-in can be found on disk.