Data are retrospective. Asking participants to rate the opening session on the last day is usually useless. Responses are judgmental or subjective. Such ratings are, of course, judgmental and subjective. What one person sees as useful another may see quite differently.
Ratings are sensitive to mood.
For example, a celebratory atmosphere in the room may improve ratings. In general, the more demographic items that are included, the more the fear of reprisal on the part of respondents. Trainees do not complete surveys. Participants don't want to complete long questionnaires from which they will not personally benefit. Ratings are sensitive to wording nuances.