I find it relatively simple. Much simpler than C++ (obviously). For someone who can write C++ and has some experience wth OCaml/Haskell/F#, it's not a hard language.
Complex is the wrong word. Baffling is a better word. Or counterintuitive, or cumbersome. If “easy enough for someone with experience in C++, OCaml, Haskell, and F#” were the same thing as “not hard” then I don’t think this debate would come up so frequently.
I mean, you can't expect to learn a new language in a few days, it'll always take a bit of work. My feeling is that people complaining of the language being hard aren't putting the effort.
My experience is that Rust is a relatively small language which doesn't introduce a lot of new concepts. The syntax is quite intuitive, and the compiler super helpful. The borrower checker was the only new thing for me. I'm not an expert at all, but my experience is that after spending 2 weeks full-time reading books and experimenting, I was able to work professionally with the language without feeling too much friction.
On the other hand, after spending much more time on C++, I don't feel really comfortable with the language.
For me, I almost never write "for loops" and "if statements" in Rust; instead I use "functional iterators" and "match expressions", which interface with the borrow checker more nicely.
For example, iterating over an array while modifying it is a common pattern in imperative languages that compiles fine but often causes hard to reason about logic errors during runtime. In Rust, such a thing would cause compile time errors. So instead you rewrite to be more functional, it compiles, and then the magic is it just works, which is a common property of functional languages like Haskell.
IMO a lot of the consternation about the cost of the learning curve is because developers haven't realized once you get past it, the benefit is your code more often is just correct and therefore you run into fewer runtime bugs. In other languages you get the code compiling faster, but then you spend a great deal of time debugging things at runtime which you didn't anticipate at compile time.