I've had three sets of extensions from different companies. None worked as well as I would have liked.
One of a set started leaking as soon as it was installed and was no good from the get-go.
One of a set worked for a while, about 2 years, and then started a slow leak.
The last ones I had were the flexible ones that were secured to the wheel with nut and bolt to hold them solidly. After removing the wheels, for one reason or another, I discovered these had been rubbing on the inside of the tires and had worn some pretty serious grooves in the tire sidewalls. They are stainless steel wrapped and somewhat abrasive, apparently abrasive enough to wear into the tire rubber. Another fail.
Now I just use the angled chucks, not as convenient, but all in all a better solution.