My point is that they still fail. Both liquid and solid boosters are a lot more mature than hybrids, no doubt. But I'm not so sure that either is fundamentally

My point is that they still fail. Both liquid and solid boosters are a lot more mature than hybrids, no doubt. But I'm not so sure that either is fundamentally more reliable. And for hybrids to mature, somebody has to use them. "Liquid/liquid engines, by any measure, are orders of magnitude more stable." What do you mean by stable? A reason to go with hybrids, at least on paper, is that they are mechanically simple but throttlable, hopefully combining the simplicity of a solid booster with the ability to throttle and shutdown of a liquid. Of course, much of that also applies to blowdown liquid systems, and I personally find a blowdown methane/lox system particularly compelling. I figure Virgin wanted to avoid cryogens, though.