How to answer questions *honestly* about some professors from Visit Days students?
First of all you should decide whether you want to convey any negative information, factual or otherwise, about these faculty members. You are certainly not obligated to discuss the issue at all. You could for instance contrive not to be present for this part of the Visit Day, or you could show up and say "I'm sorry, but I don't have anything to contribute in response to that question. Maybe you should ask someone else."
In my opinion though, absenting some very clear and serious negative consequences for you, you should provide feedback on your experience, because as you say,
Because nobody had warned me before I started working with these professors, I ended up wasting two years in grad school, in my opinion, a very, very high price to pay.
You go on to write
I would like to be very honest with the visiting students. At the same time, my field is quite close-knit, with all top professors collaborating with those at other places. If these visiting students end up not joining us, I'd not want them to remember me as the person who bad-mouthed these famous researchers.
Telling the truth about your own experiences is not "bad-mouthing" anyone.
How can I honestly answer questions about these professors without coming across as bitter (which I admittedly still am)? If I just state the facts, well, even those are going to be coloured by my viewpoint, and will therefore be biased.
I suggest that you plan out what you will say, or could say, in advance. Indeed you should concentrate on factual information. You should also remind them that this is just your own experience, and that others' experiences might be different. This should encourage the visitors to seek out information from others, which is good.
I also don't think "bias" is the right word. Your answer to the question "What's it like to work with Professor X?" is inherently about your feelings on the matter. If you had a negative experience, then that's the correct answer to the question as far as you can provide. It doesn't mean that others will have the same experience -- and again, you can emphasize that in your response -- but it is your take on the situation.
What is the alternative? Dishonestly describe your experiences and thereby lure further students into bad experiences? That's not helpful to anyone.
Provide the information needed but do it with positive remarks. Avoid negative statements and anything that would be embarrassing if quoted in public. Example:
Prof. X has an ambitious research program. This group attracts students who come in with extensive prior research experience.
Prof. Y is much sought after as an invited speaker. Due to his/her conference travel commitments, s/he tends to attract students with a high degree of self-sufficiency.
Prof. Z is a relatively recent arrival in the department. S/he provides a solid training in lab techniques and a strong suit is guiding students in defining their research question.
This profile is actually pretty common in academia, and those professors typically know that they're not good advisors because they're not interested in being advisors.
Keeping that in mind, your statement can be informative and accurate while not being derogatory:
Prof. X is a great researcher, with many publications in top-tier journals. He has a very busy schedule, and cannot dedicate much time to advising. He can only supervise highly motivated and autonomous students who aren't expecting to see him more than once every 6 months.
- It gets your point across.
- It makes it clear for the students if their profile would fit.
- Prof. X will be thankful for you not to send students in need of much supervision.