Compared to keeping it on earth, or putting it in the ocean, where there are also many things that can go wrong? Yes, launches can explode, but it's very rare, getting rarer and presumably these rockets could be launched from small islands where if there's an explosion the fallout would mostly be over the ocean.
I once read that 1 kg of plutonium burning up in the higher atmosphere would be enough to kill the entire population of the earth. I'm not enough of an expert to know if that's really true, but I suspect there's a good reason why experts don't consider this a good option. Or at least not yet.
