So I heard that you can block punches with the long guard (especially overhands), so does that mean that you can block any type of punch with the long guard?
Obviously, no kind of guard helps against every type of punch. The long guard is particularly well-suited to limit the tactical options of an opponent, covering a lot of the possible punches. But it has its limits, especially as soon as people slip past it:
When using the long guard, you must watch out for overhand rights, hooks, and low kicks.
On the other hand, as you know these limited options, you can watch out for them.