If you want a pure DPS melee job, SAM outshines the others by far. However what you heard about NIN being medium DPS support is really not true. I would say NIN is right behind SAM, perhaps tied with MNK damage wise. It's true they were doing the least dmg of all melees in Heavensward, but they definitely do a lot more since Stormblood. I would say they're probably the second highest melee dps job atm, but they bring more to the table in group content compared to SAM in the form of Trick Attack and enmity management with Smoke Screen and Shade Walker.