I know it Depends on The Quest
like
(you need to Protect someone Important that can help too Destroy Alduin but he got killed then you need to Defeat on your own)
(Protect an Important Town before it got Destroyed)
Edit : And by Failing a Quest i mean when a Quest from a Main Quest is Failed it closes some options to Complete the Main Questline
But also Opens Other Options to Complete Main Questline
So then There is another way of Completing the Main Quest
Something like that makes the Main Quest harder and more of a Challenge if some of the Quest are Failable