There are problems with sand for some species of snakes,a nd some of lizards, HOWEVER some species of both lizards and snakes thrive on a sand based substrate.
Firstly, with pythons, any specimen in the Morelia family have large open pits under their bottom lip to sense heat. Sand can often become caught in these pits, causefriction and inflaming, and in worst cases, can become infected and possibly threaten the animals life. Also with pythons (im not sure about vens with sand unfortunately
) the sand can be hard to digest, a few granules wont do much damage, but if the fed item of food is damp, and it picks up alot of sand, this may cause a problrm or 2 with your snakes digerstive system. However having said this, the Aspidites branch of pythons thrive on sand, especially the Womas.
Now, with lizards, as far as im aware, sand can damag their claws..? but i am not entirely sure on the details, how ever again, some species thrive on it, like accies and sand monitors.
Hope this helps