Not that they are more fragile as with health issues but they don't tend to hide as much as platy fry do and therefore they become a snack faster. Platies are absolute kings in hiding when small in my experience.
Also, the blonde fry guppies are for some reason very attractive meal compared to the grey ones. Platies of course grow bigger faster and they get roundier so after a couple of weeks max they don't tend to get eaten anymore by the adults.
Edit: There will be of course no difference in survival rate if all the fry are moved to a tank with no adults. In fact none should die if the tank is cycled and the filter intake is secure. My fry are almost grown now, but the two platies that I saved out of curiosity and put in the fry tank with the guppy fry could not be seen in days sometimes, even at feeding time. I know their hiding spots so I kept checking if they are alive actually.
My other tank has female adults guppy and platies together and at the moment there are at least 10 growing baby platies I never wanted or intended to save. It was funny watching them when they were very small appearing out of knowwhere to snatch the falling left overs of food.