As of such, Mega Salamence and Mega Rayquaza have many striking resemblances. But here are the main factors that kept Mega Rayquaza from staying in Ubers:
- Powerful, can literally 2HKO and OHKO the whole of Ubers after a set up chance.
- In the cases it opts Swords Dance and not Dragon Dance, it's got a nuclear Extreme Speed.
- Horrifyingly durable. Actually survives a Mega Mewtwo Y's Ice Beam to the face and OHKOs back.
- It's ability aids it's bulk a lot, helping it set up Dragon Dances, Swords Dances, Substitutes etc.
- The only mega to hold an item. Basically death incarnate because of this.
- Ability also makes it the best (and probably only) safe counter to Primal Groudon, which is also a threat in Ubers.
As of such, bulk, ability and the potential to hold an item are what differentiate Mega Rayquaza from Mega Salamence. These factors are huge, as a Choice Banded Mega Rayquaza is bound to hurt more than Salamence.
Mega Salamence definitely makes a splash, but comparing it to how much Mega Rayquaza wrecks Ubers, is like comparing a puddle to a sea (maybe not that much; but you get my point). So no, Salamence isn't taking up the top notch position that Mega Rayquaza held in Ubers just yet.
Hope I helped!