[jupp0r]

Dynamic scoping moves a bunch of correctness checks from compile time to runtime. It basically introduces all the problems that come with shared mutable state across different functions/methods. It becomes hard to reason about who mutates state where/when.

[wffurr]

Exactly. I read this:

    Ergo, dynamically-scoped variables are shadowable side-channels that can influence the behavior of a function

And thought that is exactly why not to use dynamic scoping. It makes every function impure by default.