In my experience there is a difference. 50 IBU of FWH, 50 IBU of hop bursting and 50 IBU of early boil additions do not seem the same to me in the same beer. They are all good practices that have their place in different styles. I would worry less about the estimated IBUs to a recipe than how the hops are arranged in the recipe to create the best bitterness and flavor/aroma for the recipe.My mantra with any brewing calculation is that the best use is to give a baseline for a recipe so you can compare apples-to-apples for dialing it in. Once you brew a recipe, then you have an idea of what 50 IBU's taste like for that given recipe and you have a target to adjust from.