it seems to me that it's all by chance, do alfa really know what part is at fault or is it just trial and error?
my first thought about the tensioner being metal is that each tooth could cut into/wear the rubber belt out quickly but then again both camshaft pulley's are metal so maybe not.
i think that camchains and tensioners have come on a long way since then, my mate who is in the trade tells me that most new honda cars, along with other manufacturers are now fitting camchains to their engines.
I hope so.. the chains will eventually stretch with mileage but shouldn't need changing due to age alone.
But if the tensioner gums up and stops keeping the chain taut, it will lift off the sprockets and either come off the sprocket altogether (new engine time) or lift unp enough not to come off, but still enough to wear away the cam-chain cover.
Is the cam chain assembly under a sealed metal cover, so that it keeps dust and grit out of the chain rollers and tensioner mechanism?
How is it serviced?
I think a chain is better than a belt (just seems more "right" somehow) but a bad one (or rather a chain with a bad tensioner) can potentially do more damage than a belt when it all goes horribly wrong.