Let's say you want the user of your service to be able to write and upload a configuration file, but you specifically don't want them to be able to execute arbitrary code on your server(s).

It would be nice to be able to statically verify that their submitted configuration "behaves nicely", but the moment you introduce any kind of Turing-complete configuration language all guarantees go out the window because the problem becomes literally undecidable. [And then you try to mitigate it by imposing limits like "kill the parse process if it takes longer than X seconds or uses more than Y megabytes of memory".]

The advantage of, say, a simple .INI file for configuration purposes is that the time required to parse it (and storage space required to remember what's in it) is O(n) where n=size of config file...

EDIT: so take the example Lua file from the original post but change the loop limits from "32" to "16777216":

  for i = 0, 16777216 do
     for j = 0, 1677216 do
        add_body({x = 20 * i, y = 20 * j, mass = -0.1, rad = 2})
     end
  end

Congrats! Your 128 byte long configuration file now requires 2^48 iterations of the inner loop to parse and generates 2^48 instances of the "body" object. That's an expansion factor of at least 2^41 generated bytes for every byte of input, which is a shockingly bad Denial-of-Service attack against whatever is responsible for parsing the evil config file.