It depends. It is safe within limits. When it comes to the basics of engine tuning, if you push the engine too hard, it can cause more stress on the engine parts. In the long run, this will cause the joints and bearings to wear out. Another thing to consider is the heat. When the engine power output increases, the factory-installed radiator might not be able to deal with the increased heat. These are some of the reasons why the powertrain warranty voids with engine tuning. The short answer is engine tuning is only safe as you do it. So, only a professional car tuner should do it.