Is it complicate to write unit tests for your React app?

Not at all!
All you need is to install jest testing framework:

npm add --D jest babel-jest babel-preset-env babel-preset-react react-test-renderer

Add "test" script to "scripts" section in package.json file

  "scripts": {
    "start": "webpack-dev-server --open",
    "test": "jest",

Now you have the testing environment configured and running.
And you can start with adding some very simple unit test
You can create it on some file which has ".test" in its name - like example.test.js

test('adds 1 + 2 to equal 3', () => {
  expect(1 + 2).toBe(3);
});

After running
npm test
the result will be something like this

Now nobody can say about you that you never wrote a single test