Smashing boxes would indicate the stregnth of the clutch rather than a weakness.
Imo that means they are good, the weakest link is what always fails. Though imo smashed boxes are almost always an issue of driving style.
More issues reported with JB? How on earth could you know, has there been a poll where the values have been normalised against the number of people buying them? This thread is almost soley about JB so unless you have made an excel spreadsheet with number of comments for and against for all brands and worked out the percentage, chances are your view is greatly skewed. More breakages could simply mean more people are buying them, might still be a lower percentage.