Here are my 2cents on the subject.
Before a BMW comes to market it undergoes literally hundreds of thousands of hours of testing in a variety of climatic conditions, from a northern Swedish winter to summer in Saudi Arabia. BMW will often change their spec. according to the predominant climatic conditions. For example, cars destined for the Gulf may have different cooling capacities than those headed for Scandinavia.
The reason for the extensive testing is to duplicate all the conditions likely to be encountered by drivers and assure that the engines, drive trains etc. are reliable under all those conditions. This represents a huge range, from -40 degrees in a Finish winter to +50 degrees in a Dubai traffic jam.
Chip tuning companies are typically small outfits addressing a niche market. They do not have the testing facilities of BMW so by definition, their customers are mainly the ones doing the testing.
ECU tuning increases power output and torque and decreases fuel consumption, sometimes by fairly large amounts.....a 50 HP gain is not unusual. How is this achieved and what are the knock-on effects? Typically the biggest gains come with Turbo-charged engines, both Petrol and Diesel and the main way this is achieved is to increase the charge pressure by altering waste gate settings. Ignition timing and fuel/air ratios are probably adjusted to allow a higher cranking pressure without causing pre-ignition.
So what are the effects? Firstly and most importantly, more heat; so the cooling system is under substantially more stress and will tend to run hotter. Since there's no temperature gauge, this is of no concern
It does however mean that if you drive to Rome in the middle of summer you may enjoy the drive there more than the drive back.
Second, the increased performance makes the car a lot faster, working the brakes a lot harder, which directly correlates to higher insurance premiums because insurance companies know that accelerating a lot harder while decelerating at the same rate means you reach the scene of the accident going a lot quicker, which is typically more expensive.
Then there's the increased power and torque's effect on the drive train. Is the clutch and gearbox built for so much more power and torque?. Apparently so, as hundreds of testers errrrr customers will attest to. The fact that the manufacturers don't agree and suspend the warranty is a minor irritation, but what would they know? Clearly their engines are highly tuneable and no one's posting about blowing gearboxes or clutches, proving conclusively that the tune is safe.
Of course there is another side to the coin. Manufacturers will alter the output characteristics of a motor to fit a car into a certain market slot....look at the 325d as an example. But I find it significant that when BMW released an M Performance tune for the 335i, it involved uprating the cooling system....and that was only 20BHP!