I'd like to say that I think MegaGross is a predominant force in this metagame not only for itself, cuz ok, it's strong but it's not outright broken as McM said, and its point of force are:
1) Excellent ratio staying power/offensive power
+
2) One of few really good offensive Steel typing makin it extremely viable
Thanks to its stats + typing + ability, it can take several hits from a bunch of offensive mons, outspeeds most of them (that can deal it several damages while fasters usually don't) and being a problem also for more defensive teams, tho stall can quite easily threaten it;
3) Quite good coverage
Yes, it can go itself through some of its checks with GKnot or coverage but as Bluwing said, tipical dual defensive cores can deal with it (remember Keldeo BW? It was defined "broken" due to its possible coverage but things like Jellicent + Amoonguss could deal with it while having other functions for the team so not fully dedicated to it), so I can see a Gliscor+Ferrothorn+BulkyWater having no issues at all with it (unless running Ice Punch + Hammer Arm+GKnot but hey seriously?).
"Hey but Greninja was banned for its coverage, why MegaGross can't be for the same reason?"
Things are actually different because Greninja could threat almost every defensive core depending of its coverage that could run every of its moves, instead of mons like Keldeo, Landorus-I and MegaGross itself that choosing a certain move they renounce to deal with other stuff that they could threat with another; in shorts they have 4MSS (sry if my english sux but I hope it's clear what I want to explain, isn't it?)
4) Its force as answer of main threats of "past" metagame (post-greninja ban) that made it being one of main current meta threats:
warning: it could seem theorymon but it's just a (personal) analysis of past metagame and not discussion on future one!
Remember early ORAS when everyone tried new toys as MegaGross itself, MegaMence (gone for good), MegaSlowbro (judged a bit worse than expected), MegaSableye (judged a bit better than expected), Mega Beedrill, MegaAltaria, MegaGallade, MegaLopunny, MegaDiancie?
Now, focus on MegaLopunny/MegaGallade, that initially dominated metagame because Greninja scared fairy usages so Azumarill/Clefable/Diancie itself were worse in the past. Even MegaGross itself was worse due to Greninja lol to the point to drop the "quickban hype" after MegaMence as many thought with the statistics leak pre-release.
So metagame was: fighting type, fairy type, greninja, sableye, steel type (megagross + scizor usually non-mega because waste of megaslot)
note that both fights and sableye are weak to fairies in general
greninja balanced the thing (also for megagross)
after the (rightful) ban, things started to go out of hand
Fairies got a huge buff once that they were the main stuff to opposing megafights offenses and megasableye defensives
so MegaGross as STEEL type was the perfect answer to this: DarkPulse Greninja was gone, it could threat stuff like fairies indeed (better than the "passive" Heatran and the slower Scizor), latwins, a semidecent answer for fasters fighting, coverage can threat stuff like grounds/waters/steels ecc
so it "balanced" the meta which is still crap tbh.
Looking more in the past, we can see how really good is Steel type, since their main representatives were suspected and banned: Aegislash and MegaMawile. Adding that their function as "antifairy" was one of their only few positive qualities for metagame, they were banned for their excessive impact on the metagame caused by other features.
The main consequence was a better meta because, obviously, dealing with them was an ass for building and playing in general but at the same time it caused a huge amount of usage of fairies: Azumarill, MegaGarde, Clefable.
Finally going to current meta, situation wont be different, and actually right now isn't, because MegaGross is so used (quite only) because it checks fairies and having really good utility.
I think main problem of ORAS are fairies, and suspecting every good steel (even not outright broken) seems not the best solution imo
MegaGross isn't outright broken? So the ban is to "fix up" meta ok? But what if this makes things go worse?
Ladder suspect would answer to this.
Let's see.