Any car without maintenance will give you problems. Only a brand new car will be problem free.
If you are buying a used car, be ready to keep your wallet open for sudden and unexpected breakdowns and maintenance issues.
Having said that BMW are no more or less reliable than others, say Japanese. The only difference is "jugaaro" approach of "neem hakeem" mechanics will not work on it. The mechanic should be used to the "German" approach. Everything is designed to a particular standard.
Personal Experience
I had a recent run-in with a neem hakeem mechanic here (happened to be Pakistani, if you are interested to know). My transmission oil got mixed up with coolant water. In the end, there was a thick coffee like "liquid" in both cooling system and gear box. I went to a Pakistani mechanic to have it checked, who happened to be the very next shop on my way. He drained the oil and without much cleanup, refilled it back with coolant and tranny oil. When I went to a qualified mechanic, he had to pull my tranny out and clean everything that sits inside of water and moisture. If you are more interested in the job, I can share with you pictures etc.
Fun Facts
Did you know the BMW engines typically run between 89'C and 98'C.
Did you know the BMW coolant pressure is about 50 psi.