As the maintainer states in the README.md the tab completion in z-fish, z-is not working.

I personally would use tab completion a lot as I work as a web developer and have a lot of directories with the same name but a parent directory that have unique names.

With tab completion as it is in the Bash version of z I can start typing the unique parent directory, hit tab to complete, then add a slash and start on the subdiretory, hit tab to complete to quickly get to a folder.

Currently the non-functioning tab completion is all in the z.fish file. After taking a look at how others have written completions in fish I have to wonder if this will require one or more files.
In fish the completions I have seen are in ~/.config/fish/completions/. So, one of the need files could be a completion file ~/.config/fish/completions/z.fish.