Is a bone marrow transplant life threatening?

There are risks. There is a risk of dying of a complication of a bone marrow transplant. The risk is dependent on whether the cells are your own or from a donor. Also, the degree of matching impacts the risk. The major life threatening complications are related to infections, organ failure and an immune system problem called graft-versus-host disease.
Yes. Your immune system must be destroyed before a bone marrow transplant, so there is always the chance of a fatal infection or other fatal complication.