I actually find horses VERY predictable.
If there is any object or animal out of the ordinary, there is a good chance they can spook. So I guess I don't believe in a totally bomb-proof horse either. All horses can panic- that's the nature of the beast.
But they are very predicable in that you know they can spook at any time, and after a while you get a feel for what is likely to set them off.
So I don't buy that they are unpredictable. I find their body language easy to read, and the things that tend to bother them are usually things that are not normal in their routine. You just need to learn to see the world as a horse. But I don't buy into the thinking that they just "snap." There is always a warning sign if you know what to look for. Sometimes it just happens too quick for you to prevent it.