Trump is easier to debate for democrats who don’t care as much about decorum as Biden. A primary debate requires much more delicacy, because you can’t all out attack the others without potentially prejudicing voters against your party’s eventual nominee, should you lose.
Biden did well in the latter scenario, because he’s very good at politicking. I think Harris would do better in the former because of her greater willingness to openly mock trump. Hopefully we get to see.