Just remember that a Devil Fruit's power doesn't come from raw strength, but from creativity. The more creative a person is, the more dangerous they can be with their Devil Fruit.
Doflamingo's String String Fruit is a Paramecia, but he handles it like a Logia. A lesser person would just use the puppet function or razor string. But Doffy found all sorts of uses, body doubles, the Bird Cage, internal healing. All this making him a dangerous force to reckon with.
Luffy is similar. He went beyond simply stretching his arms and legs, he can speed up his blood flow to go faster, he can inflate his bones for a strength boost, the Gum Gum UFO was probably the most creative post-timeskip move yet (hopefully the fight with Doflamingo brings out more insane attacks)
Of course, Haki is there to equalize everything. Otherwise, Logia users would rule the world.
