A. Understanding how to implement functions using syntactic forms is a steep learning curve. I remember running out of dplyr and having to implement a udf. Fairly unpleasant experience (enquo, !!, perhaps other unusual constructs). Felt like programming C macros.
B. "A function can decide where variables are looked up however it likes" is a significant obstacle in understanding how even basic constructs like function calls actually work. There is a non-trivial amount of hard-to-debug dark magic lurking behind every corner.
A middle ground has never been achieved. For example, `plot(expr(x), expr(y1 + y2))`, where the system limits the dark magic to explicit uses of the `expr()` construct, and `expr(x)` always means `{vars: vars(x), expr: (vars(x)) => x}`. Instead of patching interpreter environments, simply call a lambda function.