I've been using the Sapling CLI frontend with GitHub as a backend for all my open source projects for a few years now (I've also used mononoke and eden at work, but IMO those only apply to people with hundred-gigabyte monorepos). So speaking based on my open source work:

- I am absolutely loving the improved UI/UX for common operations - being able to do the same actions with fewer commands and fewer concepts to understand, and much more helpful error messages when I try to do something invalid

- The existence of some unique features (`split`, `absorb`, and `restack` being particular favourites -- IIRC people have created third-party scripts to replicate these commands for git, but last I checked they weren't as good, and they aren't installed by default)

- Having the commit log integrated with github (being able to see which of my branches match to which PRs, and whether the PR is unreviewed / accepted / rejected / merged)