They know what they like, but they are usually uneducated about educational theory and methods. Trainees have uneven comparative histories. Trainees usually have highly varied training experiences with which to compare a given course. 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.