I've been baking 40 years and never had this experience with icing a cake. First let me say that every part of the cake tastes great. The cake is tender and moist. The ambrosia filling is incredible on its own, and the buttercream icing is light and fluffy without being too sweet. The problem I had came when time to assemble the cake. Even though the filling and icing were chilled, once warmed up they became quite soft. They started sliding off the cake like an avalanche, and nothing I did would keep them up. This was extremely embarrassing because the cake was our dessert for Christmas dinner. I'm glad other bakers had better success than I did. I'd make the cake again but skip the filling and use a firmer icing.
This made a fantastic dessert for our Christmas dinner. I made the ambrosia filling the day before, with one addition: I chopped up about 5 maraschino cherries to mix in with the other fruit. The other change to the recipe was that I used an Italian meringue buttercream instead of the American style buttercream linked here. I'm glad I did, because I think the frosting would have been too sweet and heavy otherwise. The final cake turned out tasty and beautiful, and I would definitely make again for a special occasion.
I love to bake, and have made many different cakes over the years, and this is probably my all-time favorite. Although a little time-consuming, it is absolutely worth it. Note: Be sure to start the Ambrosia filling well in advance or the day before.