What is considered 'bad' in a guard clause?

Patterns and Guards are a small separate language inside Elixir (and Erlang). Compiler takes a look at the guards and patterns and generates very efficient BEAM assembly. Therefore, guards are limited only to a set of declarative functions which you can find here (for erlang) and here (for elixir)

For example,

case something do
  1 -> ...
  2 -> ...
  3 -> ...
  a when is_atom(a) -> ...
  b when is_binary(b) -> ...
end

Will generate a jump table, and switching in this kind of case will always be O(1)

Then ‘bad’ would be code that can’t be optimized as you described.

I suppose, “bad” from description means “dont’t care about internals, just use what’s allowed”