Eflin is clearly better than Pivetta or Velasquez, how that is debatable is beyond me.
Eflin had a cold spell where he was trying to figure out the kind of pitcher he was, part of the blame goes to Young who wanted him to be a power 4 seam FB thrower (seduced by that 94-96 velocity), but as become apparent, Eflin is more effective relying on the 2 seamer and using the 4 seamer as a "show me" pitch to keep hitters honest. I'd go so far as to say some of Nola's struggles has been over reliance on the 4 seamer this year and not using his excellent 2 seamer more often. But when Eflin has both FBs, his slider/cutter and changeup working, he has #2 potential.
Velasquez has an explosive FB with late movement, but little else, and when he tires, the FB loses some of its movement. His other pitches are mediocre at best.
Pivetta has a straight 4 seamer like Eflin, has tried to throw a two seamer with limited success, and while his curve can be plus, he struggles throwing it for strikes and hangs a few. I think he has potential as a setup man b/c his FB adds velocity out of the pen.