The biggest issue with the 250 is the limited low cycle fatigue life of the turbine wheels.
The wheels are integral castings. That means that the wheel is made up of a disk that is cast in one piece with the blades. The blades are not removeable. What happens is that on startup and as the engine accelerates, the rim of the disk gets hot quicker than the bore (the center of the disk). This causes the rim to yield in compression. When the engine is shut down, the disk is hot and the rim cools quickly and the disk rim now goes into tension and cracks.
To help reduce the amount of thermal stress in the disks, it is very important to cool down the engine after you land and before you shut it off. This allows the disk bores to cool off and minimizes the temperature difference between the disk rim and the bore. The shutdown procedure includes two minutes at flight idle before shutting down the engine, and to get the most out of your engine it should be followed to minimize the damage to the disks. Going from a high power hover to a quick shutdown is a really bad thing to do to these engines.
If you don't properly shut down the engine the cracks, which grow from the rim of the disk downward toward the bore, can progress to the point where the disk will fail and you will burst the disk.
The disks crack and the crack growth is well understood, so there are time limits on the disks and periodic inspectons. In all it is a safe system when the procedures are followed correctly.