Over the last couple of decades the problems inherent in polling have resulted in a great many missed calls, including Brexit and Trump.
Here's what happens: people who vote for populist candidates either distrust polling organizations (in which case they don't share their intentions) or they prevaricate; either way, their true intentions aren't captured in the polls.
Furthermore, it's perfectly possible for someone to disapprove of (or even loathe) a candidate yet end up voting for them, especially in a nation like the USA where one's political allegiance, like one's allegiance to one's favorite sports team, is an intrinsic part of one's identity. Thus Republican voters may disagree with Trump and even find him nearly as repellent and repulsive as people with an adequately functioning frontal cortex, yet still vote for him on election day because, well, the poor dears just can't help themselves. Frankly, they'd vote for Stalin, PolPot, or Hitler if any of these folk stood for election with an R next to their name on the ballot.
Meanwhile, Biden inspires practically nobody, and the Dems always eat their own rather than focus on trying to defeat the opposition.
Hence the smart money is still on a Trump victory in November, alas.