You don’t mention the series or the mileage, so any response will be very general. My family has owned a slowly evolving fleet different BMWs over the past 20+ years so I guess I am as qualified as anyone to make general statements.
A 535 X drive will probably be more reliable than a 540 or higher. If you are looking for reliability, however, a used BMW is probably not where you should be looking. My daughter’s boyfriend’s BMW has less than 90k miles on it, and it has broken down unexpectedly and left him on the side of the road three times in the past three months.