That is a very interesting question. I bought my guy because I felt safe on him, he is pretty laid back, no buck, can't see him bolting, he did think about a rear one time, got one foot a few inches in the air, but couldn't bring himself to lift the second one, I would let about anyone ride him.
A few weeks ago I was leading him in the arena, when a wind gust caught the outside door and slammed it, and he shot forward several feet, his reaction took my by surprise, and the door obviously surprised him.
All horses are living breathing creatures, so no one can guarantee that a horse will be 100% safe in all circumstances, but there are many many like Gibbs, really laid back about life.
“Never attribute to malice that which can be attributed to stupidity”