What are the reasons to not use componentWillMount in React?

Question

React

You often hear recommendations not to use this method, even the creators of React themselves report this through the console.
So, what difficulties can arise when using this method?

Answer 1

Quote from offsite

Note:
These methods are considered legacy and you should avoid them in new code:
UNSAFE_componentWillMount()

https://reactjs.org/docs/react-component.html
Do you need to translate the word legacy ?