• Pijul: patch-based like Darcs but apparently solves its performance issues. In theory this improves conflict resolution.
• Jujutsu: kind of an alternative front-end to a git repo (but not a front-end to git). Has some different ideas, like no staging area (draft commit), and some other stuff I can't remember.
• Sapling: from Facebook. Unfortunately only part of it is available. The server is not public yet (I guess it's tired up in Facebook infrastructure too much).

And it's definitely not a solved problem. Aside from the obvious UX disaster, Git has some big issues:

• Monorepo support is relatively poor, especially on Mac and Linux.
• Submodule support is extremely buggy and has particularly bad UX even for Git.
• Support for large files via LFS is tacked on and half-arsed.
• Conflict resolution is very very dumb. I think there are third party efforts to improve this.

I think the biggest issue is dealing with very large code bases, like the code for a mid-large size company. You either go with a monorepo and deal with slowness, Windows-only optimisations and bare minimum partial checkout support.

Or you go with submodules and then you have even bigger problems. Honestly I'm not sure there's really an answer for this with Git currently.

It's not hard to imagine how this might work better. For instance if Git repos were relocatable, so trees were relative to some directory, then submodules could be added to a repo natively just by adding the commits and specifying the relative location. (Git subtree almost does this but again it's a tacked on third party solution which doesn't integrate well, like LFS.)