I have to be honest and say I've never tried B2N. I just go by what others have told me.
I've heard a couple of people say they think the product is 'gritty' and don't like the way it leaves the hair.
Just recently a few have said they swear it caused their system to shed.

I'm not sure about the last claim.
It seems to me more likely that by the time the hair has faded enough to consider using it, its become brittle and the shedding would have happened anyway.

You know when you buy an off the shelf hair dye it has 2 bottles.
One is the peroxide developer. Thats the stuff that strips the hair so the colour can be absorbed.
Usually in a store bought colour that peroxide is 12%.
If you're just darkening your hair a shade or two back to its original colour, 12% is way too strong and unnecessary especially for processed replacement hair.

Anyone dying their system hair should really consider going to somewhere like Sally's Hair & Beauty and buying professional dyes and then a much lower % developers.
The staff in these places are very helpful and can advise you.