Depends on what you mean by "safe".
Could a high dose of benzos cause you to do something that leads to you dying, like crashing a care, sure. Could benzos alone kill you, probably not.
Benzo's by them self are pretty safe, you'll probably pass out way before you'd even be able to do anything harmful, however mixed with other drugs can be problems. Some swear that opiates+benzos = death, and it does slow down CNS a lot, however a bigger problem is doing so many benzo's your so retarded you shoot up a few grams of heroin at once, not knowing, and die. Alcohol is another great way to end up in a bad situation, while benzos+alcohol alone can be a problem, more likely you're going to end up being super drunk-benzoed and in turn do stupid things.
So overall, benzos are pretty safe alone, and not the end of the world to mix, but at the minimum can cause bad side effects, and if completely used poorly together could ause straight out side effects, like respitory depression