There’s a slight difference in nuance, let's see this example:

✅ „Das ist wirklich schwer.“

1. „Das“ refers to a specific thing previously mentioned or pointed at.
2. It’s like saying: "That (thing/situation) is really hard."
3. More contextual and specific.

✅ „Es ist wirklich schwer.“

1. „Es“ is more general and abstract.
2. Like: "It is really hard."
3. Common in general statements or when no specific thing is mentioned right before.