Prevailing sailing conditions are part of his numbers?
You're right, because boats vary in strength and stability. Probably only the Southern Ocean racers are at the cutting edge of "seaworthy" and "fast", because they get rolled, dismasted and sometimes inverted, and yet only a couple of sailors (by definition among the world's best...most of us couldn't even do the qualifying races) have died or gone missing.
For the rest of us, prevailing conditions play a big role. Alex, up to 40 knots I would take your boat, but higher than that I would rather be in mine. Part of that is simply that a pilothouse is a better place to be when the spray is like nails hitting you in the face...another part is that your boat is engineered for speed and CAN be broken (remember your exploding boom?). If you have the right boat in the wrong conditions, all bets are off. If you have the right boat in harsh but survivable conditions, but only one guy knows how to sail the boat, after 36 hours, that guy will collapse from fatigue.
For anyone interested in the topic, I recommend strongly a book called "Rescue in the Pacific", about a 1994 "weather bomb" that hit a cruising rally between New Zealand and Fiji. Fatigue, and not seaworthiness, were the biggest problems: People who simply couldn't continue (or who were injured in capsizes) had to be rescued from boats that were seaworthy enough to eventually be found intact or run aground.
Here's some sample pages. I found the last chapters (what worked and what didn't) very informative.