This was discussed a bit before, I think (it's on like page 7 or something). Mega Metagross still has too much bulk, speed, power, and coverage for my taste. Aero is faster and stronger, but Metagross has enormously more bulk, better coverage, isn't weak to rocks, doesn't miss, and isn't choice locked. Scizor looks similar at first, but it's half as fast, needs to commit more of its moveset to utility, and doesn't have many coverage opotions. It's only niches over metagross are bulky SD and defensive webs. Sylveon isn't very comparable to Metagross imo, since it has basically no bulk, coverage, or speed.
It might not get many new toys to play with, but STABmons is a much more centralized metagame than OU is and Metagross' best quality is that it can pick and choose what it wants to beat with its enormous movepool. Sure, it can't run all of its coverage moves at once, but you can just run Heavy Slam and then pick 3 of Zen Headbutt for Rotom and Venusaur, Ice Punch for Lando and Chomp, Hammer Arm for Heatran, Ferro, and Skarm, and Grass Knot for M-Slowbro, and let its five teammates deal with the few pokemon that it can't outspeed and 2hko. Other strong options are Thunder Punch, EQ, Bullet Punch, and Rock Slide.
The best defensive check to it that I can think of is Bulky M-Scizor. Although I'm sure LCass will come in here and let me know that Bronzong, Doublade, and Mega Steelix also wall it, but i mean...
It's insanely hard to revenge kill Metagross in this metagame, especially with Thundy's gone. Scarf Landorus, Scarf Volcanion, Scarf Heatran, and Talonflame are basically the only things that can outspeed and reliably OHKO it. Serp and Pidgeot can put it to sleep, but if Metagross wakes up early then they're dead. Gengar speed ties, ScarfChomp's PBlades is only a 50% to OHKO, Weavile can't kill it, ScarfTar can't kill it, Scarf Keldeo can't kill it, Band Aero can't kill it, and FakeSpeeders don't do anything to it. None of these pokemon can switch in, since they all get OHKO'd by heavy slam or some common coverage move.
It can also mirror its old SG set with Agility to clean up teams EZ, given that it's extremely resilient to priority, tanks almost every hit in the game, outspeeds everything at +2, and can hit for ~75% against every pokemon.
I agree that it's not completely off the charts broken like it was with SG. But the insane offensive roll compression that metagross provides makes it hard to justify not bringing it on any team that isn't some sort of stall. It can wallbreak with its huge coverage, out-tank almost everything and provide a great fakespeed resist, it can revenge kill with high speed, power and access to bullet punch, and clean pretty easily with just a few threats weakened. It lures its own checks with its coverage, and if you want to play a slower build, you need to bring 3+ checks to it if you don't want to get bopped by just being up against the wrong coverage set. In OU, it's a bit of a jack-of-all-trades-master-of-none pokemon, but with heavy slam it really masters almost all of these rolls.
I don't really think retesting metagross is the right move right now. I would rather take a closer look at things like sylveon and aerodactyl, which are really strong in the current metagame.
