Reliability in this context means that the camera will record and the footage will not be corrupted on the media.
Film sets are very rough on equipment and things break all the time. Sometimes one films in very harsh conditions that anre either very cold or very dusty and hot. Often you don’t have the luxury of being able to repeat a moment or you have travelled to very remote locations so having gear that will continue to work is worth paying a huge premium for.
The Arri sensors and imaging pipeline also offer the best overall image quality. This means it can handle very high dynamic range scenes better than all other cameras. For example the new Alexa35 sensor can record 11f stops of information above middle grey. Most prosumer video cameras can record above and below middle grey around 12 stops total with most of the information in the shadows.
It also means consistent image quality in different shooting environments. Arri has very sophisticated cooling to keep the camera sensor within a specified temperature for consistent noise performance.
Because Arri make more than just cameras it also means that the camera fits into the whole professional eco system and synergises with other pieces of filmmaking equipment like the Arri wireless focus systems, camera remote heads etc etc. Meaning you can focus on the hard bit which is creating amazing stories.