It's actually a combination of both factors, the board flexes due to the heat cycles and torsion load from the x-clamps, and the lead-free solder used on the BGA is less forgiving of this flexing than traditional solder, it's more susceptible to developing micro-cracks with the flex.
As for how long the 'fix' lasts, it's down to luck as much as anything. You never know how damaged it is, you may be lucky and have it still working 12+ months down the line, but you may find it only lasts a matter of days. I've done hundreds of the things, and I'd say maybe 20% have the fault re-occur.