I think this situation is very weird considering IT WAS WORKING before, so ... i would suspect the dealership honesty.
Now going to the chase onto the problem itself... the ABS unit is a regular DC Motor that spins to change the pressure on the wheels and some electro mechanical valves actuated by solenoids controlled by the ABS ECU.
Now, what can be wrong? i think basically if you never change the fluid, the fluid passages valves that are controlled by the solenoid coild be clogged, i would take the ABS unit apart, sumerge it in some kind of solvent, and they with a air compressor, clean the passages.
By the other hand, if the problem is not the fluid passages but something electrical, you can always reparir the DC motor on it, rewind it, etc, i think is very hard to find a ABS unit that cannot be repaired.
I know A LOT of FZ6s, i have a S2 ABS my self, and i have never experienced problems and dont know anyone with ABS unit problems. So... to say that is a VERY COMMON PROBLEM... i think is a over statement.
It could fail... yes, off course, anything could fail, but i dont think is common and i think it could be a easy fix.
I would disassemble entirely, sumerge it in some form of paint thinner and they with pressure air on all passages, try to remove any clog.