I failed the first few times I tried Italian Meringue Buttercream too, but am very glad that I persisted. It opens up a whole new level of frosting options. I learned by using RLB's recipe in the cake bible. I agree that Italian Meringue buttercreams tend to taste buttery, but certain flavorings and especially chocolate cover that up, and you can't beat the smooth creamy texture. But because of that tendency, I no longer make IMBC. Here are some thoughts:
1) I have tried a number of IMBC recipes. They are all very close to the ratio of 5 whites per pound of butter. I agree that the recipe you have is incorrect and that adding more butter may have solved the problem (but you didn't know that).
2) the temperature of the butter (and the room) has to be correct. It is not terribly precise, but if the butter is too warm or too cold it will not come together. I agree with the post above that the butter should be brought to room temperature and mixed up so it is smooth and creamy before adding to the meringue, but not sloppy soft. If it is not coming together and the room is really warm, I put some blue ice packs around the bowl, and then it comes together. If you suspect the butter is too cold, just let it keep running in the mixer. Sometimes it takes a while to come together. Especially if the recipe is on the lower end of butter amounts.
3} if you refrigerate the buttercream for later use, it must come COMPLETELY to room temperature before rebeating. If it is too cold, the frosting will separate into chunks of butter and clear sugary syrup that splatters all over the place. (personal experience!) At that point it is hopelessly lost.
4) I now use Swiss meringue buttercream in place of IMBC. I have found the SMBC to be less buttery tasting, and, for me, simpler to make. This type of buttercream brings the sugar and whites to 160 degrees before whipping the whites and adding the butter. My favorite recipe uses 3 cubes of butter (3/4 pound) per 5 whites which is less than the IMBC. It is my go to recipe for most cakes. For kid's cakes and sweeter cakes I just love the Fluffy American Buttercream you can find in the recipe section. It is made with pasteurized egg whites and hi-ratio shortening. It is smoother and creamier and less sweet than the traditional American Buttercream, but sweeter than the IMBC, which the kids (and some adults) seem to prefer. The FAB and THE SMBC are both so good that I often can't decide which one to use, and I mix them together!
5) The flavorings make all the difference. The recipe I use for SMBC (which I always double to have on hand) is 1/4 cup vanilla per 10 whites! I also think liqueurs add a very rich flavor to BC and you don't taste the alcohol. I usually add an extract and liqueur to SMBC, but leave out the alcohol in kid's cakes of course. I have great cake recipes, but it is always the frosting that gets noticed.
HTH, and good luck!