Some issues with "useful" computational work is that it needs to be computationally easy to verify the result (a hash function is but other work may not be), also it may not be crptographically strong so someone could come up with some method to solve the problems much more efficiently than others and make the network insecure.
Some issues with "useful" computational work is that it needs to be computationally easy to verify the result (a hash function is but other work may not be), also it may not be crptographically strong so someone could come up with some method to solve the problems much more efficiently than others and make the network insecure.