The issue isn't really a 'failure rate' type of thing. The problem is that the screws are tightened against the plastic body of the NSU. Engine heat and heat cycling causes the plastic to soften slightly and collapse a little under the pressure of the screws. Once the plastic has deformed the screws are no longer tight. Most riders find 'finger tight' screws. A few riders find a screw is missing or one comes out during an oil change. Mine were barely finger tight when the bike had 300 miles on it.