Cuevas definitely get robbed! Often times more than once. It's a tough business. I just had lunch with one of the biggest cuevas downtown Buenos Aires. And he told me a story of armed gunmen going in and robbing the place. Down town the big cuevas have a system where there are cameras and you only can come one person at a time behind 2 locked door system and even with that they managed to get robbed once he told me. He said probably with the help of police or people working in the building.
I asked him if they ever got robbed delivering money and he said they also got robbed. About $300,000 USD. I also have a friend that has a cueva and last year for the first time in 25 years he also got robbed. A $250,000 USD loss. With so much cash you have to be careful.
I to this day wonder why there aren't more robberies from Western Union or outside of cuevas. I figure the police just put the fear of God in these people if they do anything to affect tourism they are toast. I formally owned the largest property management company in Argentina. With hundreds of properties and tens of thousands of guests each year. Never once did I ever hear of a guest getting robbed outside of Western Union or a cueva.
These days I just use a cueva that delivers money to me. They will give you a good rate and as long as you are exchanging at least $400 to $500 USD they will deliver it to you free. I mostly just do that now. Time = Money.