I am far from an expert in these matters so I can only quote others who seem to know more.
Peter Schiff sounds like a gold salesman at times (he is afaik), but he makes the argument that rather than fixing the issues that caused the bubble that led to the Great Recession, the Fed and stimulus managed to only make it bigger. Now the market is burning itself up with expectations of economic growth under Republicans, but all they are doing is growing the deficit and putting the US deeper into a hole that it can never dig itself out of. Soon the US will be unable to acquire more debt and we'll do another round of quantitative easing, which could spiral into hyperinflation as we're forced to repay our debt by printing money. This will be terrible for the US because we've become a nation of debtors and consumers. He believes the coming recession will be worse than 2008 and that we are in the largest bubble the US has ever experienced, possibly never having even escaped the 2008 recession in the first place.
He believes all this will come due before the end of Trump's term, probably within a year or two. I've heard other famous investors say a recession is likely to begin next year.
These might not be in order of when he spoke:
https://www.youtube.com/watch?v=1Fd58n4HhAM
https://www.youtube.com/watch?v=WD2zcyfwdJ4
https://www.youtube.com/watch?v=5DuHWGh2GgU
https://www.youtube.com/watch?v=03Ke9rIYGsc