Language support: You can implement extension traits on an integer so you can do things like current_node.next(v) (like if you have an integer named 'current_node' which is an index into a vector v of nodes) and customize how your next() works.
Also, I disagree there is 'zero safety', since the indexes are into a Rust vector, they are bounds checked by default when "dereferencing" the index into the vector (v[i]), and the checking is not that slow for vast majority of use cases. If you go out of bounds, Rust will panic and tell you exactly where it panicked. If panicking is a problem you could theoretically have custom deference code that does something more graceful than panic.
But with using indexes there is no corruption of memory outside of the vector where you are keeping your data, in other words there isn't a buffer overflow attack that allows for machine instructions to be overwritten with data, which is where a huge amount of vulnerabilities and hacks have come from over the past few decades. That's what is meant by 'safety' in general.
I know people stick in 'unsafe' to gain a few percent speed sometimes, but then it's unsafe rust by definition. I agree that unsafe rust is unsafe.
Also you can do silly optimization tricks like if you need to perform a single operation on the entire collection of nodes, you can parallelize it easily by iterating thru the vector without having to iterate through the data structure using next/prev leaf/branch whatever.