When Zoro first used Enma it pulled so much Haki out of him it almost disintegrated his arm, and sliced a massive chunk of the cliff clean off. That was with him using only enough force to try to cut a tree in half.
On Onigashima, Zoro, with one Sword, sent a flying slash more than halfway across the the skull, and cut the horn off like butter.
On Dressrosa, Zoro said using a flying slash against Pica at the distance he was at, would be no good, because it wouldn't be able to knock Pica out. That Enma slash he used on Onigashima would have knocked Pica out at twice the distance. Again, this was with just one Sword.
The reason is obvious here, because with Enma, he is capable of using much more Haki than without it. Once he masters Enma, he'll be able to use that amount of Haki with all his Swords.