I picked a composite tank up really cheaply here in Brisbane about 15 years ago from BBQs Galore. The sales guy said it was cheap because locals were 'suspicious' of them, because not made of steel. I saw the advantage straight away. No corrosion, much lighter, (held 15L and weighed less than a standard 10L steel tank), and see through meant no being caught out with an empty tank. Sorry, but disagree with the gauges used on steel tanks being much use.
I Loved it, but sadly when the 10 yrs was reached it became clear that to pressure test it, it would effectively be destroyed, because the valve is sealed in, and cannot be removed. No-one would fill it unless it was certified, so I reluctantly parted with it, but hated doing so.
I suspect the 10 yr expiry talked about is purely arbitrary, and not based on any real scientific or real-time testing evidence, as the substance they are made of is similar to fibreglass resin, so probably would last almost indefinitely if out of the sun, but I guess they had to decide on some time limit to keep the lawyers/insurance co's happy.