People claim the timings are different but there is 0 evidence supporting that.
This unproven rumor has been going on ever since the TechPowerUp article came out. There were cases on their forums of around 17 or so cards failing down the line a few months later. Of course, without a proper controlled experiment you can't tell if it was due to the 6970 bios flash or whether the card itself was just faulty.
No one really knows and no one really cares to find out.
I do not remember seeing anyone with problems straight off with the unlock shader bios in comparison with the 6970 flash.