At a glance, ZeroFS appears to be an overlay filesystem above a VFS. It looks like it abstracts the underlying filesystem configuration/complication and creates a simplified, unified configuration that could allow you to be more dynamic underneath.

It also appears to support a great deal of IO and permission complexity, which makes sense as it refers to itself as a filesystem. It also purports to be extremely fast. In the end it looks to function like an actual filesystem with all that entails.

rclone (my first time hearing about it) appears to be a much simpler solution with different design goals. It looks more like rsync in most instances, or a user-space link to a remote filesystem in others (like ftp or smbclient). The mount capability, while similar to zerofs, looks more like something much simpler and targeted for less-sophisticated needs.

Not saying one's better than the other; merely that they really look to be serving different needs/goals while having some overlap.

rclone is MIT license.

ZeroFS is AGPL with commercial license available.

https://www.zerofs.net/licensing