I would imagine that like anything else, a mold can "go bad" for some reason but I have been casting now for over 50 years - I have molds of all makes and have had some Lee molds that I bought years ago - some with thousands of boolits cast out of them. Mine are mostly single/double cavity so I'm sure that may make a difference - never had a six hole Lee. I have a Lee DC 358-158 TL SWC that I've cast probably close to 6K out of and it still drops them like butter.
For the price, Lee molds can get the job done and keep on doing it if taken care of. But then again, I had a new one that I sent back and never cast in it as the cavities were misaligned and the pins not installed correctly - should never have left the factory. Like tools, cars, etc. - all depends on how you treat them and if you take care of them. I don't bang on my molds and have never sued a sprue whacker on any of them - aluminum or steel molds - but I've seen plenty of old Ideal/Lyman molds that look like they've been beat on with a ball peen hammer - but they still cast well. Aluminum? Yea, softer and things can happen . . but like everything in life, everyone's mileage will vary. -)