He is so outrageous he is hard to plan for, his lies are easily rebuttable, but it takes a lot of facts and a good memory to do so, which cuts back on Biden's time to make his own case and makes his response sound boring and defensive. Also, putting together a cogent rebuttal requires time and practice, while lying can be improvised on the spot. The advantage is all Trump, especially now that Joe is older and slower. Biden has the advantage of reason and experience, but he's not Hillary or Obama and it takes time and preparation to fully exploit that in a way that is memorable for an audience. In a debate, a pithy, unexpected lie or insult has much more impact and is a lot harder to counter than a complex, reasoned riposte. You don't reason or finesse with a mugger, you have to overwhelm him with sheer violence.

Perhaps later, when the conversation is analyzed, Biden will be shown to be the winner, but on live TV, Trump will appear confident and dynamic, he is, after all, the showman.

I don't think it will go well for Biden.