i think that the best substrate for bearded dragons, especially when they are young is tile or the non stick shelf liner. (the kind without holes) I wouldnt trust that bark bedding. one of the most common ways that pet bearded dragons die is of impaction. I just dont trust lose substrate. it can get into their food when they are eating. I guess if you feed your dragon outside of the cage and it is an adult it would lessen the risk. It just a risk i wouldn't take. It seems that it would be hard to know for sure that you got all the poop out when he poops. non stick shelf liner is non absorbent (the kind without holes) and the poop just wipes right off if you catch it early before it dries.
No bark. That's a bad idea. If they happen to swallow a piece, then it would rip up their insides. Just imagine what a proportionately sized piece would do if YOU ate it. Stick with non particle substrate. My vote is always for tile.
Agreed. Loose substrate is hazardous. I'm using tile, which is pretty easy to clean; take out the soiled piece and wash it off. However, I put newspaper underneath in case anything leaks in between the tiles...