In an unstructured interview the interviewer chooses questions on the spot, follows the conversation wherever it goes and forms an overall impression at the end. Two candidates for the same role may be asked completely different things. One might spend twenty minutes chatting about football and holidays while another is questioned closely on their experience, which makes fair comparison almost impossible.
Unstructured interviews persist because they feel personal and give interviewers confidence in their own judgement. That confidence is often misplaced, since impressions are shaped by rapport, appearance and similarity. Teams that want to keep a conversational feel can still add structure: a core set of job-relevant questions for everyone, notes taken during the interview and scores agreed against a rubric before candidates are discussed.