"Dangerous" is relative. In most cases, you won't sustain much harm by doing so, although the items in your backpack might get crushed. I have had it explained to me before by former Jujitsu teachers that this is one of the reasons some of the defenses involve turning a flip to escape from a hold rather than a throw, because it prevents you from rolling over your sword. A breakfall may also be a better choice in some situations. Learning multiple options lets you react appropriately for the situation.