lack of funding,
Here I think you can be straightforward. Probably the faculty are just as disappointed as you are.
administration being uncaring towards students,
I don't know exactly what this means; as a grad student I never tried to attract the attention of any administrator. If you have any specific problems you might bring them up.
general departmental atmosphere and culture among the grad students, general culture of the school,
If you're talking with an old friend privately, sure, bring it up. If you're meeting students at some big admit weekend, then you would piss people off by saying anything, and indeed there's not much reason for you to: prospectives will get a chance to observe the atmosphere for themselves.
poor location, etc etc.
Griping about this is pretty common. (Among faculty too!) I don't think it's especially frowned upon to complain about this, at least if your point of view is widely shared or if you are speaking to prospectives in private.
Indeed, I remember one student at one school telling me: "The only positive thing I have to say about the campus or the location is that there is adequate parking."
Also my school is the only top school in my field that has a "rotation system" for picking advisors, and that is generally framed as a benefit, and a reason to choose the school.
Tread a little bit more carefully here. "In retrospect, I believe that I would have fared better without this system; my experience was blah-blah-blah. But this is a system that our school touts, so you might talk to other people too, to get a sense of what would be best for you."
That said, if this were some kind of admit weekend, I'd consider inventing an excuse to be elsewhere for the day. If not many current grad students are willing to show up and talk to prospectives, that will itself serve as the warning you want to offer.