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.
Red V-Raptor S35 XL has 16.5 stops of dynamic range with 250-12,800 ISO. ARRI Alexa 35 has 17 stops of range with 160-6400 ISO. Both use rolling shutters, both have native ISO of 800. Alexa has better low light noise reduction at higher ISO, but Alexa is sharper and has better dynamic range in low light since it uses 8K to 4K down sampling.
The real difference is in the colour straight out of the chip and how the workflow is for DIT on set.
THe major difference I think is that Arri is already easy to use and slots in to the Hollywood human knowledge base and workflows while RED was mostly used by indie filmmakers, documentarians, Youtubers, and Silicon valley people (if you work at Apple in the US you can buy RED cameras cheaply through company benefits). This pretty much created a different culture of what images should look like around the two cameras. an Alexa camera sets you back close to 80K and a fully equiped RED sets you back about 44K. So its easier to buy it and use it while hollywood rents it on the day for the shoots.
You can get a RED to look like it was shot on Alexa and vice versa in post processing today, but the people who work with the different cameras have different cultures of what is "cinematic" image.
btw this is a good comparison https://wolfcrow.com/red-v-raptor-s35-xl-vs-arri-alexa-35-wh...