Why is Windows 10 so unstable?

The reason why Win 10 is unstable (in the sense of having many severe bugs) is because of all the new code Microsoft has written.

Which Windows 10 update is causing problems?

Windows 10 update disaster — Microsoft confirms app crashes and blue screens of death. Another day, another Windows 10 update that’s causing problems. … The specific updates are KB4598299 and KB4598301, with users reporting that both are causing Blue Screen of Deaths as well as various app crashes.