In my opinion a written guarantee on genetics is worthless. It simply takes too long to prove whether it is or it isn't what the breeder said it was and there are too many variables that can either help you or hinder you in that proof.
About 13 years ago I bought a pair of het yellow ghosts. I didn't request it, but the guy I bought them from included a written guarantee that they would produce visual ghosts within 5 years. I raised them up and bred them at three years old. It didn't take. The following year I bred them again and got a small clutch of eggs but hatched no ghosts. The year after that I got a clutch of eggs but they all went bad. At this point, the guarantee did NOT prove out, but is that the breeders fault? Is it my fault? Or is it just the luck of the draw? It was kind of a moot point anyway because a few years after I bought the snakes the guy went out of business and disappeared from the ball python scene so I wasn't going to get my money back anyway. The following year (year 6) I produced a clutch of 3 nice looking ghosts out of 5 eggs, not bad odds after all.
In the end it really boils down to 'do you trust the breeder or not?' Because if you don't, no piece of paper is going to make it all better.