Based on rebuilding a cobra I had for a short while, plus what others have said in previous topics.
The issue seems to be that as the idler doesn't have a seperate bar to run on, it's all moulded as one piece, which makes it the weakest point in the gearbox. As long as you don't run too hot a motor and the slipper works it should be ok (check the mardave site for the 'new' slipper if yours doesn't have one).
But it was more of a comment about how cheap they are!
There's lots of useful info in here:
http://www.oople.com/forums/showthread.php?t=27465