If you doubt your teacher, do everyone a favor and choose one of the following:
1) Tell him that you don't think his shihonage is at all effective and for him to prove it on you. Attack him and resitst as strong as you want. Try letting go if you can. Report back here afterwards.
2) Stop wasting your time studying with a teacher whose effectiveness and teachings you doubt.
3) Train for twenty years and then ask yourself if shihonage is not effective.
Shihonage is one of the techniques which have caused deaths in aikido training.