I can only swear by royal icing. I have moved some monster cakes and have taken them through some rough terrain. And have never had them slide.
I have never really played with candy melts, so I dont have much to offer on their dependancy.
edited to add.. I also make sure I use a good amount and smear it around on the board. In my area I cant take a chance on a cake sliding. over kill is the only way to go!! 
RI works great, although sometimes I'm lazy and just use BC. But I can't say that I'm delivering monster cakes! Usually the biggest cake I need to transport is a double layer half sheet, and the weight of that thing keeps it from sliding. I'm more worried about the top layer shifting during transport.
I wonder why your cakes started sliding? Was it warm? Were they stacked cakes that needed a center dowel?
Cakeythings, it only happened twice. Both times, it was just a 2 layer fruit filled cake. I took all the necessary precautions I've learned here (stiff dam, not too much filling, settling etc...). The first time it was the top tier and the second it was the whole cake. It wasn't damaged it was just touching the sides of the box and smooshed the bc; both were iced in bc.
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