From a toxicity standpoint they are safer. Bismuth can be consumed in medicine (Pepto Bismol). I work in toxicology and we are worried about the "big four": lead, cadmium, mercury, and arsenic. These are from the Q3D guidelines on toxicity. Tungsten and Bismuth aren't on the list at all. Nickel is a unique concern, since it's so dang useful in metal alloys, and a high percentage of people have an allergic reaction to nickel particles.
Edit for a plug: Don't throw away NiCd batteries folks! Recycle them properly!