Yep, adding or removing an element would invalidate existing pointers to any other element in the hash table. This is generally regarded as a good thing if your elements are stored contiguously in the hash table, because a resize would cause any existing pointers to dangle. This should be true for C, and might be true for C# if you're using `struct`s which put the data inline (memory's a bit fuzzy on C#'s rules for references to structs though, maybe someone can chime in).
This new approach still requires us to be mindful of our data layout. Not caring about data layout is still definitely a strength of GC and RC. I'm actually hoping to find a way to blend Nick's approach seamlessly with reference counting (preferably without risking panics or deadlocks) to get the best of both worlds, so that we can consider it for Mojo. I consider that the holy grail of memory safety, and some recent developments give me some hope for that!
(Also, I probably shouldn't mention it since it's not ready, but Nick's newest model might have found a way to solve that for separate-chaining hash maps where addresses are stable. We might be able to express that to the type system, which would be pretty cool.)