The evidence for poll unreliability is all the recent elections whose observed results did not match the predictions of their associated pre-election polling. And the predictions have missed basically in only one direction (always overestimating Republicans/underestimating Democrats).
there’s no evidence for “the polls are unreliable” in any historically special way either. even if you do the simon rosenberg thing and only count the longstanding reputable ones. you can theorize on why This Time Is Different and might even be right. but you’re just vibing, sorry
One recent example, though, was the NY-3 special, which if you look at aggregators was polling super close — around 1 percentage point lead down the stretch — but turned out to be a very comfortable D +8 result on Election Day.