Honestly the main reason UFCS is good is when you don't "own" the type your function would need to be a member of. Most common one I run into is the "to" function from the stdlib. You can do this:

    import std.conv;
    
    int foo=3;
    string bar = foo.to!string

    MyCustomInt to(T)(T v) if(is(T==int)){
        return MyCustomInt.from_int_value(v)
        // or however you actually do the conversion
    }

    int foo=3;
    MyCustomInt bar = foo.to!MyCustomInt;