This won't be an exhaustive answer, but I can asnwer a little. Also note, this is purely based on my own experiences with Rust (and Pony) and from what I've hea

This won't be an exhaustive answer, but I can asnwer a little. Also note, this is purely based on my own experiences with Rust (and Pony) and from what I've heard from other folks as well. Both are difficult for folks just learn the languages to grasp. When I tried doing async programming in Rust, I found the borrow checker especially hard to deal with. Pony's reference capabilities, once you understand them, apply quite naturally to async code. This is important as most actions in Pony involve sending asynchronous messages between actors. There are certain patterns of passing around isolated regions of memory that are a pain in the ass to do in Pony that are very easy to do in Rust (once you know how to work with the borrow checker). It's something we are aware of on the Pony and are working to address. Reference capabilities are part of the type of variables. This can make writing your own generics really hard unto you have a really good grasp of the type system and reference capabilities. It's the steepest learning curve in all of Pony. Reference capabilities exist only at compile time but, something about how people think about them leads people to wanting to be able to match on them at runtime. I suspect this is something about how we teach them. It's a bit of a hurdle for some folks and can lead to some confusion early on. Someone who has spent more time writing Rust and helping people learn Rust could come up with a similiar list. From time to time, Steve Klabnik and I have talked about similiarities in how problems folks have in learning the two languages. I prefer reference capabilities to the borrow checker because reference capabilities more closely align with how I think about data safety.