I did not say it was the best, but it is certainly a better option than almost any substrate sold at big chain pet stores. Crushed walnut and calci or vita sand are well known by the bearded dragon community to contribute to high mortality rates in young bearded dragons, as these substrates are much too easily ingested. Burrowing is not essential to a healthy bearded dragon lifestyle, and dig boxes are only recommended for gravid females. The 3 top substrates ANY bearded dragon breeder will tell you to use are reptile carpet, tile, or paper towels/newspaper.
The holding humidity/saturation issue isn't a problem for bearded dragons, since the humidity is supposed to be very low anyway and misting is hardly ever required. A lot of keepers don't even keep water bowls in their tanks, since the daily (or every other day) soaking is more than enough to keep a desert reptile hydrated. Holding bacteria is only a problem if you let the carpet sit and gather poo for extended periods of time without washing it, which my husband and I do weekly.
Reptile carpet is a very safe substrate for bearded dragons, chosen for many reasons besides it's ease of cleaning.