If you say that something is not for the faint-hearted, you mean that it is an extreme or very unusual example of its kind, and is not suitable for people who like only safe and familiar things. It's a film about a serial killer and not for the faint-hearted
If you describe someone or their behaviour as faint-hearted, you mean that they are not very confident and do not take strong action because they are afraid of failing. This is no time to be faint-hearted