I have just recently done 2 brews using Williams Oatmeal Stout extract as part of the recipe along with 6 to 8 lbs whole grains. We just sampled one batch bottled on 11/25 that would be about 3 weeks old and it is wonderful.
The batches did have very high final gravities, 1.037 and 1.036, using US04 and Wyeast 1084 yeasts. Original gravities were in the 1.093 and 1.097 and the ales have a sweet and full bodied taste. Both were in fermenters over a month prior to bottling at 68 - 70 degrees.
I researched this across the net and many folks state that the Williams does not get down to the low final gravities predicted by brewing programs such as BeerSmith which I happily have. Calculations show 1.020, 1.021 with the actuals over 50% more!!
Williams does not share the exact contents of the extracts, in this case I think there is probably lactose giving unfermented residual sugar impacting the final gravities.
I know I have some excellent tasting stouts while using the Williams and really don't think there are 90 bottle bombs in the basement.
Somewhat supporting this is an all grain milk stout started 9/2 and bottled at a high final gravity of 1.040 on 10/14 using US04 at 68 degrees. After sleeping on it, I dumped it back into the fermenter while it contained the 6 oz of priming sugar and added Champagne yeast. It was finally bottled on 12/2 with a final gravity of 1.038. Five gallon recipe contained 1# of lactose......