> programmers must manage pointer offsets to each heap structure

I'm sorry but I don't understand this statement. Can you re-phrase it?

> I would be more interested how they plan to handle IO effects or if they want to omit that.

I'm not sure exactly what you're asking for, but see the Coroutines, Effects and I/O pages in https://github.com/google/wuffs/tree/main/doc/note