Congress has always had the power to set minimum and maximum penalties. The existence of these ranges is the source of the sentencing disparities the dissenting justices cite. But to establish an elaborate set of formulas to determine sentencing is to deprive judges of the ability to take into account mitigating circumstances and other factors which are generally lumped into the general category of "justice." Furthermore, a lot of the problems encountered in Federal sentencing could be eliminated if the Federal government didn't try to duplicate State laws.
The formulas are a pain in the rear for jurors, I was a juror in a federal criminal trial and had to answer five additional questions after the finding of guilty on the count(s). The instructions were vague and the questions badly phrased, but they were mandated by law.