Not OP, but I’ll bite.
The weird open stache {} brackets that are permitted when the last argument of a function is a closure. This is a ridiculous feature, combined with ResultBuilder, made the language worse. And I will assert that both of these awful features increased complexity for the sole purpose of making SwiftUI syntax approachable to JavaScript devs.
On the other hand, I hate react-native and prefer to not live in a world where all my software is glitchy and slow, so I understand Apple’s motivations. React-native is a threat with hip modern syntax. Apple still used the icky MVC with the crusty old CALayers.
I just hope it was worth it!