If you work with a contractor and they tell you that your AC is guaranteed to be problem-free, find a new contractor. Seriously. You deserve to work with honest people. No contractor with any integrity would make such a false claim. The fact is that all air conditioners need repairs from time to time.
Are there ways to limit the need for repairs? Of course. You can and should schedule professional air conditioning maintenance. You should change your air filter regularly, too. That’s a more important task than many people realize. Even then, you’ll still have occasional AC repair needs.