If no references are involved, writing unsafe Rust is significantly easier than writing correct C, because the semantics are much clearer and easier to find in the documentation, and there's no insane things like type-based aliasing rules.

If references are involved, Rust becomes harder, because the precise semantics are not decided or documented. The semantics aren't complicated; they're along the lines of "while a reference is live, you can't perform a conflicting access from a pointer or reference not derived from that reference". But there aren't good resources for learning this or clarifying the precise details. This area is an active work-in-progress; there is a subteam of the Rust project led by Ralf Jung (https://www.ralfj.de/blog/) working on fully and clearly defining the language's operational semantics, and they are doing an excellent job of it.

When it comes to Zig, the precise rules and semantics of the memory model are much less clear than C. There's essentially no documentation, and if you search GitHub issues a lot of it is undecided and not actively being worked on. This is completely understandable given Zig's stage in development, but for me "how easy it is to write UB-free code" boils down to "how easy is it to understand the rules and apply them correctly", and so to me Zig is very hard to write correctly if you can't even figure out what "correct" is.

Once Zig and Rust both have their memory models fleshed out, I hope Zig lands somewhere comparable to where Rust-without-references is today, and I hope that Rust-with-references ends up being only a little bit harder (and still easier than C).