* The Bernie Sanders constituency has diminished from what it was in 2016. That is evidenced by how badly he lost Michigan to Biden, after winning Michigan easily in 2016. Even accepting your premise that Bernie Sanders supporters don't like Joe Biden (and there is no empirical evidence to support that), it doesn't appear to be a cause for concern. The vast majority of Bernie Sanders support is in California - a very safe state for Biden, to say the least.
There is sound logic and reasoning to believe that Donald Trump's support is being undercounted in polling data. This isn't the result of a left-wing bias either. No pollster wants to ever be proven wrong later. It's the fact that people who are lesser educated, not as well-read, and who live in rural areas, are less likely to have ever taken the time and effort to have completed a polling survey. They also tend to be more reluctant to cooperate with pollsters who contact them by phone. This significant segment of the American population also tends to overwhelmingly vote Republican in presidential elections. In turn, they went heavily for Trump in 2016. However, this segment of the population isn't substantial enough to overcome a double digit lead, which Biden is showing in most of the current polls.