Each method may engender a specific bias. Collecting data online may save a lot of time for you as an evaluator, but people unfamiliar with the use of computers may be discouraged.
The opinions of the interviewees often diverge diametrically. Do not forget to ask respondents for characteristic attributes according to which the results can be broken down.
The quality of the evaluation depends to a large extent on the reliability of the respondents. Do not overload them with endless questionnaires.
Are your questions comprehensible? Pilot the questionnaire with a representative group.