I don't believe that using the correct frequencies could be harmful to good cells, but using improper frequencies probably could be. Also there is the concern that a lot of Rife machines are just basic signal generators and only output one frequency rather than the collection. They're supposed to output a carrier wave with the correct frequency overlayed which creates a multitude of sidebands, and it's the sideband frequencies that kill bad cells.

Another machine to look into is the Lakhovsky MWO (multi-wave oscillator). From my limited understanding it also uses frequencies but instead boosts the good cells rather than destroying the bad.

I'm no expert when it comes to the science behind these machines, but seeing two devices that work similarly but slightly differently made me come up with the following (note I could be wrong): The body resonates at a specific set of (likely multiple) frequencies when it is healthy. When it starts resonating out of that range, it becomes unhealthy and disease can take hold. You can either eliminate disease, or amplify health by adjusting what frequency the body resonates at by applying external electromagnetic fields which can interfere either constructively or destructively based on the current frequency of the body and the frequency of the external field.

You know how if you take a bunch of pendulums linked together at their base and start them going at different times, how they'll eventually want to sync up? This is like that. Place the body in a field that's vibrating at the frequency of good health, and the body will naturally start to sync with it and vibrate at the same frequency. Then disease naturally falls away since it can't survive in that environment.