OK, I'll try to explain:
DDW, or wing 'n' wing, the wind is from behind. There is no apparent wind, i.e. the windex is not lying to you. The sails are out perpendicular (or so) to boat centerline. If you are smart, you are using a preventer of some sort on the main. You steer a course that keeps the wind coming from behind. Not much chance of a capsize unless you are in big chop/waves, bury the bow, and pitchpole (more likely on a multihull), or you get a really fast shift of over 80-90 degrees - not very likely . Life is good until you run out of lake or whatever you are sailing on. Then you either heave-to and stop, or bear away on a reach.
When sailing close-hauled, as close to the wind as you can, shifts just tend to luff the sails and/or make the boat round up (assuming a correctly tuned rig). A big gust from the side can make the boat feel like it's going over, but easing the sheets and hiking out *should* level her out again. This is assuming one has reefed the main BEFORE encountering such conditions ...
To my mind, sailing on any reach - particularly beam or broad - presents the highest risk of capsize, because again, capsize is most likely to occur any time the boom swings uncontrolled from one side of the boat to the other. On a reach, as the true wind is coming from further astern, shifts can more easily catch the main leech and throw the boom over ...
I guess what I'm really getting at here is that a capsize is much more likely to occur when the wind is from off the stern than off the bow.
Please tell me if I am way off base on any of this, and go ahead and have at me, but this has been my experience so far with The Red Witch, and we've had no capsizes yet (touch wood!) ...