I disagree. I've interviewed an abnormal amount: roughly every 8 months since getting my first job, and for more than one job at a time.[0]

Based on my experience, the best interviews have been the ones with the most detail shared up-front. When I say best, I don't just mean my own performance has been best, but I also feel that the interviewer has been most satisfied and that they have learned what they meant to learn.

The reasons for this are several-fold. In order to prepare a candidate well, there must be a well-structured interview process. This means that each interview should have some explicit goals - information about the candidate which the interviewer intends to elicit.

It is painful for everyone and a waste of time for the goal of an interview to be unclear to one party (and many times so if the goal is unclear to both). My favorite interviews have been at several FAANGs, which were highly structured. I knew up front what the interviewers would be looking for in broad strokes, and how the interviews would be structured to elicit that information. I knew how technical acumen would be ascertained.

I much prefer this over walking into a room with no idea what will happen. I do very well in those interviews as well, as I have spent much of my career in professional services, where part of my job description is to be always ready to project competence and to rapidly build rapport with new clients and potential clients. I also have much experience in interviewing to lean upon. Nevertheless, this is still a worse interview process, as I have to spend time figuring out what the interviewer needs to learn.

You may object that a good interviewer should be able to guide the interviewee. This is correct on the face of things. However, any people are bad interviewers, because there is rarely much training or prep for them. I have had plenty of successful interviews where I have had to lead the interviewer, rather than the other way around.

Additionally, there are very few jobs where job requirements include being quick on your feet and being able to rapidly (aka within an hour or so) ascertain what someone needs and how to give it to them. If you are looking to fill such a position, no-prep "genuine reaction" interviews might be a good fit. If, on the other hand, the position requires analytical thought and deliberation over periods of time greater than an hour (or whatever the length of your interview is), then such a no-prep interview seems a poor method to learn whether the interviewee has the skills you are looking for.

All this said, I do not like interview processes that have large homework requirements. It is one thing to inform the candidate of the structure and broad content of the interview, but another thing entirely to request that they do extensive work. My most extensive interview prep (excluding homework assignments) has never been more than a few hours, even for FAANG interviews with much structure and lots of prep materials shared by the company. If you can't find a few hours to prep for interviews, you're probably not taking them very seriously. And if you can't fit two or three hours of prep into a period of one to two weeks, then you've got bigger problems in your life.

[0] Note I haven't changed jobs this frequently, but I've always kept an eye out for interesting opportunities. I would interview companies to see if I like them or their job offer. I would frequently bail out early if it didn't seem like a good fit. Some I went for offers and used that to negotiate. Some I went for offers just to learn what comp packages look like. Some I went for offers to get the job.