I think what Scott ZHP was trying to say is that the cooling system on most BMW cars from this era is unreliable at mileage beyond this. I know from my experience that the plastic parts that BMW uses tend to get very brittle over time, and that expansion tanks, radiator hoses, heater core hoses, and water pump impellers have a tendency to crap out, starting at around 60,000 miles. The hotter the environment, the shorter the lifespan. It's pretty much the reason why ECS Tuning sells cooling system refresh kits pre-assembled and ready to order as a complete system.
The E53 isn't all that different from my E46, I've had to replace every single component on my cooling system since I purchased the car 2.5 years ago. Doesn't mean that these BMWs are unreliable, just means that there are weaknesses with certain systems, and cooling is one of those systems.