The ubi backend means I can use nearly any binary published on GitHub without needing to worry about a flake or nixpkgs. Just get the binary directly from the author. Same for many of the other backends https://mise.jdx.dev/dev-tools/backends/.
Tasks are very powerful, they can use dependencies, flags, args, completions, and watches. Also can be defined either as strings in the config or point to shell scripts or even python/node/whatever scripts.
The shebang trick is very fun for writing portable shell scripts https://mise.jdx.dev/tips-and-tricks.html#shebang
The fact that mise doesn't depend on nix is both a blessing and a curse. I have less tools available and I don't have the power of a package manager, but on the flip side I don't need to deal with the idiosyncrasies of nix.