yes there is a sacrifice on the front driveshaft...
advantage is that the fronts already have a driveshaft that is made for higher angles, i know mine has a CV joint at the top of it, since the blazers were such a shortwheel base they needed this stock.
so lowering the t-case cross member will make the angle worse for the front, but even the factory driveshaft compensates for it....i run a 4 inch lift and it works perfectly....
a typical CV joint will compensate for up to 6 degrees without significant wear (some say more, some say less, thats the # i go by)
you dont need extra spacer in some cases, just move the spacers from the outer holes to the inner holes between the frame and crossmember....now if you have rust like i do, new 2 inch spacers will need to be made...