ummm.... no....
They dont do similar things at all. Literally the only similarity they have is Bullet Punch. Thast it.
Mega Scizor is a bulky defogger, bulky set up sweeper, or a set up sweeper. Metagross is a wallbreaker that s near unwallable. Scizor loses to a lot of mons such as Keldeo, Fast Heatran (and even SpDef if Sciz is bulky, and Zard X, to name a few. I wont go into detail about Megagross because I made a long ass post here, so ill let that post do the talking.
tl;dr Megagross and mega scizor are different, and Mega Scizor being in one rank should in no way influence Mega Metagross being in S where he belongs.
I don't think it's fair to say Bullet Punch is the only similarity. Just because Scizor can sweep doesn't mean it can't hit hard too. They share similar bulk and wallbreaking capabilities, but they do their job a little differently. Scizor doesn't actually have trouble with stuff like Skarm, Slowbro, Lando-T, Ferro, and Cress. There are less things on Stall that check it compared to Metagross, altho you can feel free to prove me wrong if I'm missing something. It also doesn't have 4MSS and destroys all the fast megas after an SD.
Also, as far as teambuilding goes, would you use Mega Metagross and Scizor on the same team? Probably not because they pretty much fill the same role IMO.
To be clear, I'm not saying Mega Metagross is not deserving of S-rank. Just wanted to see what people thought about it.