I think you'd have to distinguish between racing multihulls, which do go bottom-up quite frequently, and cruising multihulls, which are usually carefully designed to avoid this. A look at some of the major designer's websites will show how they calculate stability using hull separation, height of rig, sail area, etc, so that a capsize due to wind pressure is only going to happen as a result of some freak event (a williwaw off a steep mountain, for example). A well as that, multihulls tend to act in a raft-like manner, skidding sideways on a wave, rather than tripping over a fixed keel (Tystie, with her lifting boards, acts in this way in a steep sea, which makes her feel safe and comfortable).
Can you give some page numbers in "N to Eureka" where squalls are mentioned?
Is there perhaps some over-dramatisation on the part of some sailing narrative writers , to sell more copies? I've certainly had to reef in a hurry, on occasion, in mountainous areas, but seldom in the open sea, where it's usually quite easy to see a line squall coming, even at night. Of course, having junk rig helps. When a reef is a simple matter of easing the halyard, you just mutter an imprecation and do it, and another reef in five minutes time is no big deal either. If you get caught unawares, with your nose in a good book, then letting go the halyard will save you from an embarrassing blow-down.
We have the example of China Moon - low junk rigs on a cat - which spent time in Patagonia, where williwaws are common, and subsequently sailed non-stop from Brazil to Tasmania through the 40s, to demonstrate that a well designed ocean going cat is safe enough.