One way or the other, a washback is a "necessary evil", and it is a form of discipline. Some would call a washback a second chance. In the U.S. Air Force, I knew many learners who were washed back because they failed an exam. Sometimes, these same people were washed back repeatedly. Others failed so badly, they were discharged or kicked out. I hope this helps you with your research. Thank you.
Washback could have a positive significant impact if the assessment results in favorable changes in language learning and teaching strategies. If the changes are desired and encourage students to adopt a deep approach to learning, washback could be effective and useful.