Personal anecdote, but I am pretty certain most of the recent CI build failures I have seen in the past few weeks in projects of mine have been due to some breakage in GitHub.
It is more than 50% chance, to the point where I don't really care much about failures. It usually means I need to re-run/retry the build at some later point.

If not GitHub, also often it's something being offline or throwing server errors.

Maybe around 5-10% are real issues.