I've been asked this when i interviewed ppl but i never gave them the truth and I've never gotten any honest truth when i was on the other side also.
People in the interview are under scrutiny and don't want to be responsible for driving away candidates
Really good people are hard to come by in this market (need to be not just smart but also easy to work with, communicate etc), so you better don't express dissatisfaction, otherwise they go to a competitor.
It's really hard to get honest opinions before accepting an offer. You have mostly gut feeling to go by, maybe join a team for a coffee break, this kind of thing. A direct question "what do you like least about your job?" could work too (it's kinda "what's your biggest weakness?" in reverse), but often times that's also just used to upsell or BS you, e.g. "the food here is too good, I'm getting fat". But you can try to read between the lines of course.
If you accept the above as true, then you can start to reach out to team members at the company and ask them what the work environment is like.
If the work environment lines up with your values, then you'll probably be fine, even when the hiring manager moves on to greener pastures.