BD is not anywhere near the hardest if traveling in Y if you're traveling below a certain threshold of miles per year, because for some programs it would be impossible to ever even reach *G. For example someone flying only 39K per year could never even reach *G with some programs but with BD your status/qualifying year reset at each level. So even though you need more than 50K to go from zero to Gold it is still doable with such a low level of flying per year. It will take one and a half years, but it is doable. Other programs which are based on a fixed calendar year aren't as flexible in this respect so aren't of any value to some who don't fly more than say 50K in any given calendar year.