Are idioms useful in IELTS? Sure, they are. Especially if your aim is to push your score higher than 7.0.
Snow & Ice
They are absolutely applicable for the SPEAKING part, as here you are to demonstrate your fluency in English, your ability to speak like a native, be kind and lively. Moreover, they can make your speech more coherent.
On the other hand, idioms are not in favour in WRITING. Being