What is its range and speed? Does it work same in very low or very high energies? Does temperature affect it? Does it change at relativistic speeds?
Etc. The tool we have to probe it is currently only entanglement and secondarily tunneling. Maybe some calorimetry and charge detection.
The mental benefit of MWI is that instead of worrying about both entanglement and observation/wave function collapse, you only need to worry about entanglement because what Copenhagen describes as collapse is really just entanglement of the observed system with the observer.
The "observations" that other interpretations cause endless confusion about are simply interactions between the observed system and the observer, but "observed system" and "observer" are concepts without a corresponding physically reality. They're simply labels assigned by humans.