Calci-sand is the least safe substrate in my experience. I've assisted in the necropsy of about 600 adult beardies that died as a result of calcium sand impactions. I've seen over 700 that were impacted on playsand, but when you consider how very small a percentage of beardies are actually kept on the calcium sands it is actually by far the more dangerous of the two.
Michele Hewitt did an excellent study on calcium based sand and here is her article:
http://www.pythons.com/calcium.html