Unlike real products, training can be tested with instruments. Even the questionnaire survey can't be 100% objective, and the effect of training is related to many factors.

See if there is an agreement before training on what to do if the effect is not satisfactory, and then get the proof of dissatisfaction and communicate several times more.

You can also ask the other party to make some compensation, for example, if it is an institution, you can talk to another teacher, or just answer questions that students don't understand.