No matter what way you choose to look at it, Enma is a PU. Without Enma, Zoro is weaker. He NEEDED Enma to be able to train and make his haki usage more effective. If he switched to a lower grade blade without any special qualities, he doesn't have this opportunity to grow stronger. Just like Gears are PU's for Luffy. He still needs to master them to reduce the negative side effects but they are undoubtedly powerups for him. By giving Zoro Enma, Oda is telling us that Zoro was not ready to face the opponents he would face in Wano without it. Without Enma, he undoubtedly loses to them that much is clear. The same goes for Sanji and his RS.