everything has issues -- plastic can come unscrewed, metal can get loose...on all the felt can get dirty/gritty.

For absolute surity, only use Leica cassettes with hand-rolled film, I guess -- I actually use my Watson loader to load Leica cassettes since the dial on the end includes the ability to close the cassette's gate after I'm done.

Of course, this means you have to use an older M-Leica which some of us do not find to be a burden at all. The nice thing about Leica cassettes is that, even 50 years later, they still work and never, ever, scratch.