Many processors still survive if one processor dies. Add another processor to expand gradually. Both can wreak major havoc. Spread the workload over the available machines in the most cost effective way. Software is more complex.
Communication network can lose messages and become overloaded.
Security can become a problem since a computer is most secure if it minimizes network connections and computer is retained in a locked room. Array processors with one instruction unit that fetches an instruction and then commands many data units to carry it out in parallel, each with its own data. Good for vector processing. A group of independent computers, each with own program counter, program and data. If one CPU writes to location 44, all willsee the new value.