Actually what they have to do with ABS is hook up their Digital Technician equipment to cycle the ABS pump and valves while they bleed the brakes. Air gets trapped in the plumbing and valves of the ABS system, and old fashioned bleeding techniques will not get that trapped air. If you take your ABS equipped bike to a dealer for brake service and he doesn't hook it up to the DT for bleeding, you need to insist he do it right or find yourself another place to do business. This is not new, it is not rocket science, but unfortunately some of what passes for "technicians" at the local Harley stores aren't really qualified to do much more than wash bikes.
As for loss of braking power on the rear brakes, the ABS should not affect braking power right up to the point where you start to lose traction and the ABS kicks in. The master cylinder, caliper, rotor, and pads are the same as the non-ABS version, and normal braking should be the same. If the rear brake isn't performing properly, by all means get the bike checked out by a competent tech. Could be something in the ABS, could be air in the system, could be contaminated brake pads or rotor, etc. In other words, basic diagnostics are called for, not guesswork.
Jerry