Obviously not every Leo that is kept on sand is going to become impacted and die, but it is possible and it does happen. A Leo kept on tile, reptile carpet or paper towel just isn't going to become impacted (from substrate) so it is a much safer bet.
My vet is diabolically opposed to it, he yelled at me once about other people and went off on a tirade about it's use saying far too many Leo's die from sand and that it is 100% preventable. He couldn't understand why folks went against his advice and ended up with impacted, dead Leo's and were surprised about it! He wasn't yelling at me personally but felt he could vent about it to someone who didn't use it.
I don't know how your pet/reptile shop is but do know that whatever I'm told at any of the ones near me is usually wrong