This also applies to FM microphones. If privacy is important, FM will not give you that kind of security. For simultaneous transactions, you can have up to 32 channels at the same time. The Infrared System has been around long enough to become a standard in public places. Some churches choose this kind of system because the theatre or concert hall down the street uses infrared.
Most infrared systems seem to be compatible.
The infrared system is secure for privacy and it is very good quality sound. Some of the drawbacks are light and line of sight. In some churches with large windows, the sunlight can add noise to the audio signal. Relocating the emitter can help but a second emitter is often the solution. Another problem concerns the elderly.