I have never heard about a BMW braking down maybe with a 20 year old model that has not been looked after. Any car from new if it is well maintained will be reliable. Every car manufacturer needs to make money from car parts and repairs in the end. If you are buying through Motability its not something you have to worry about. If your buying private you just buy whatever car you can afford to run and maintain.
I had an X1. Nightmare. Auto box problems. Back to main dealer 3 times. After 6 months and a range of courtesy cars lease terminated. Breakdown happen to all manufacturers. A night end marque makes no difference.