Open questionnaires allow the respondent to answer in as much detail as they want.
Practical: cheap, easy, lack of researcher effects, quick to reach conclusions, low response rate, misunderstanding of questions .
Ethical: infomrmed consent, anonymity, questions may be sensitive.
Theoretical: Detailed data, reliable, large sample, detachment and objectivity, participants may lie, someone other than th eintended participant may answer, call of rapport.