The problem is you need to ensure it's not bad in any possible configuration from an accident. This is hard to do. Will the energy release at criticality drive the material into an even more critical configuration? Such "autocatalytic" systems were considered for bomb design, but weren't chosen because of the large amounts of plutonium needed. But a fast reactor might have the plutonium of hundreds of atomic bombs.
Edward Teller famously warned about this is a nuclear industry trade publication in 1967.
The only fast reactors I'd trust would be ones with fuel dissolved in molten salt; it's hard to see how that could become concentrated in an accident that doesn't boil the salt. But such reactors have their own problems, in particular exposure of reactor structures to intense fast neutron fluxes (not as bad as in fusion reactors, but worse than LWRs.)
