Luffy developed the best way to use his devil fruit power. We were able to see some devil fruit users who could only use the basic attacks, and they were laughing his rubber ability out, but when you combine it with haki it looks invincible.
The rubber allows him to widen his skin and to create balloon like muscles, that he covers in haki and than everyone is watching him in amazement. Gear 2. was taking away his lifespan, but he got even that under control. He still can’t make full armament haki like Vergo but I bet we will see that too in several years.