Static binaries, or dynamic binaries whose project has documentation on what dependencies they need, are better than appimages. This is because appimages are a container with the actual files inside, creating a layer of abstraction, and appimages require libfuse to work.

Imagine the case in NixOS, where dynamically-linked binaries don't work out of the box. You can patch or package these binaries, or just quickly use something like steam-run to emulate traditional Linux bin and lib paths, it works. With appimages, it won't work unless you already have libfuse in your system, so you have to extract the appimage first.

Still, flatpaks as the only official alternative isn't great for many reasons, and CLI/TUI programs are out of the equation. What is better is the devs distributing unpackaged binaries, jars, etc, and optionally flatpaks. Either way, Nix's repository is huge so I don't usually feel the need to run anything that isn't a nix package.