Is Arduino reliable for serious tasks, such as medical machines? I'm just getting started with Arduino, and it has a LOT of uses, however, I would like to know how reliable is it? Can it be used for medical purposes (Measure vital signs, or to help surgeons perform surgeries), if it depends on the board, which one is the most accurate and reliable?

I know most robotic tasks will rely on the quality of the servo to interpret the Arduino commands, and sensors, and even the coding/logic, but we can replace/recode all of those things, but when it comes to the main board it gets more complicated
You should NOT trust using an Arduino for life safety measures (medical purposes). 

This discusses the reliability of an Arduino pretty well- https://electronics.stackexchange.com/questions/15535/reliability-of-the-arduino-platform-for-industrial-use