I can understand why some people think Mega Gross is banworthy, on paper he certainly is. In practise however its another thing imo. He is still realy good in practise no doubt, but not banworty good imo.
What many people mention here all the time about 2hkoing the whole meta and lacking save counters and switchins is true but it rarely works that way in practise for several reasons.
Yes he can 2hko everything, but only with perfect prediction. Things like Rotom-w, Ferro (just 2 examples of the top of my head, there are many more) etc can all be 2hkoed on the switch, but only if predicted correctly. If they switch into another move Meta cant 2hko anymore and is forced out/has more trouble dealing with them. This problem is enhanced by the fact that Metagross has so many possible switch ins, bulky waters, bulky steels, Lando-T, Garchomp, Gliscor, Mandibuzz there are so many mons out there that can switch into 3 of his moves without getting 2hkoed that you will usually find 2 or even 3 possible switch ins to Metagross on a well built team. Yes there is some risk involved here, and yes the risk is bigger for player facing Mega Gross but that can be said for most mons.
Another issue related to that is his coverage. Meteormash, while powerful, has awful coverage and isnt realy spammable. Most of the time Zen Headbut is the best option to spam but thats resisted by many things as well. And if you go for that and a resist, like some steeltype comes in, you will miss out on the 2hko in many cases.
The point is while it might look difficult to switch something into Metagross in theory, i never found it that hard in practise.
His bulk is certainly amazing for an offensive thread but with so many things running ground/fire moves for coverage alot of things have SE moves to heavily damage him. This adds to the prediction problem mentioned earlier. Its not even about the switch in, if you got your Meta in against Clef against a team with Rotom-w and Heatran, what move do you pick? Go for Mash and the other two can come in for free. Go for Zen and Heatran can come in and force you out or Clef stays in and uses Flamethrower (not quite sure but doesnt full def Clefable even live one meteor mash?). Metas bulk certainly helps to compensate that to an degree but from my experience its rare for him coming in more than 2 or 3 times until he is worn down to the point where he cant do much any more. At least against non stall teams.
The number of all set counters is realy small but the number of checks that can switch into 2 or more of his attacks without beeing 2hkoed is huge.
And the people mentioning how +1 from mash can win games should also mention how missing with all those 90% can lose you games. Especially considering that Mash isnt a good move to spam with so many things resisting it.