not sensible because not doing well or achieving that which is intended

unrealistic

not producing the effect(s) desired

useless

(of a person) wasting time, energy etc in their work or duties

not working well; wasteful

(of) an animal without a backbone or spinal column, e.g. insects, worms

weak, useless

not valid

ineffectual, valueless

not able to

not having talent, skill