Depends on if they are set up with a double trigger. Here is a decent explainer, but you can research more about RSU double triggers that can help avoid taxes for large-but-private companies: https://www.parkworth.com/blogs/pre-ipo-tech-giants-using-do...

What this does mean, though, is that until the second trigger is hit you haven't technically vested the RSUs. So you get around the taxation but there may be additional conditions on your equity.

Basically - make sure you read the stock plan