UFCS only works well when the language also supports function overloading, which is a questionable feature by itself because it greatly complicates resolution processes and constrains other type system features. For example, function overloading in Rust will definitely conflict with traits (and Rust trait system is already complex enough). UFCS increases the complexity on top of that, and the resulting convenience is more or less marginal in my opinion.
Yes, it would be great to have a way to turn a method into a function. That doesn't necessarily mean that they have to be unified---an explicit conversion is enough.
UFCS is not the feature to call anything like `a.f(b)`, it's to make all functions callable the same way. Calling a method like `A::f(a, b)` is also UFCS and Rust does have that.
I'm specifically referring to UFCS as in D and Nim (possibly more). Rust's `A::f(a, b)` was also called UFCS in the past, but it is a distinct syntax and the term is no longer used [1]. The UFCS in question would look like `f(a, b)` instead, and conversely, you may be able to call an ordinary function `f` with `a.f(b)` as well. Rust allows neither of them.
Yes, it would be great to have a way to turn a method into a function. That doesn't necessarily mean that they have to be unified---an explicit conversion is enough.