The downside is whenever you want to do something quite standard in UIKit that doesn't exist yet in SiwftUI. The "carousel" that we have at the start of the app is a UIPageViewController in UIKit, but we had to re-implement it ourselves in SwiftUI.
Also there are weird bugs and crashes. A lot. I think in the end, the productivity gain from SiwftUI evens out to the same productivity as UIKit due to the time spend on those bugs/limitations.
We also planned to build widgets very soon, and having the same code base (they are SwiftUI only) is definitely going to be a plus.