It's a cool feature which enables some really nice APIs, but it also has the potential to break your mental model when reading code. For example: x <- f(a)

It's a cool feature which enables some really nice APIs, but it also has the potential to break your mental model when reading code. For example: x <- f(a) y <- g(x, z) vs y <- g(f(a), z) It would be extremely unexpected if the result `y` was different for these two expressions, but you can't know for sure without reading `f` – since it could be using NSE. This applies to any innocent-looking function in any library you use – imagine trying to debug an issue caused by something like this. It's also awful for performance. Allowing functions to peek at their arguments at any time is one of the things that makes R virtually impossible to optimise effectively. Of course, libraries like dplyr also show that it can be used to great effect in the right hands.