Having played EU I think longer than any NA that's left (that being 3 and around half a year) With the 15thYR. I would say EU's are far better at blocking and working together than NA's. You don't really see a lot of ego during groupfights or 1v1s while working together that you see in NA. However, I think in terms of chambering and over all stabs NA's are superior. As many who are left in the NA competitive scene knows how each other operates we are always adapting to find new ways to fuck each other.
In summary. EU's and NA's are in my opinion on equal footing melee skill wise.