I don't think the cocconut fiber is a good substrate for dragons. It can still cause impaction, as the fibre isn't digestable and it tends to hold water and swell up. I have heard of created geckos chocking and dying from this stuff several times. Also, it holds moisture, creating alot of humidity in a cage, which isn't good for bearded dragons. Also when dry, it tends to be dusty.
Washed sifted playsand has it's risks as well but in general, it is the safest loose substrate available for bearded dragons. It has it's cons as well, as if injested in quantities, it can cause impaction. Also, it can harbor bactria, as it's vertually impossible to scoop up all the soiled dirt when they poo.
Though shelf liner isn't as natural as other substrates, it has teh benefit of having no risk of impaction and is easy to clean thoroughtly.
I know of other dragon owners who have made 'sandboxes' for their dragons to play in, with the rest of the cage shelf liner or papertowel. Thus, there is a place the dragon can dig and have fun in, while greatly reducing the risk of sand being eaten, either accidently while hunting for crickets or during dragon 'tasting' of their territory.
What it all comes down to is what you decide is the best substrate for you. Make it an informed decision based on what you like, the risks each substrate poses and what you want to spend on substrate.
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PHLdyPayne