4 Move Syndrome is a good problem to have. As it makes your opponent have to guess which coverage moves you're running.
I never understood this argument. In no way is 4MSS ever a good thing even though it makes your opponent guess. When would you ever
not benefit from having everything you need in one moveset? If your opponent
knew your moveset, but you have all the moves you need, it doesn't matter that they don't need to guess, they're screwed either way since you have every move you want.
Rather, 4MSS gives them the
option of guessing your moveset, rather than knowing that your set covers everything you want it to do and resigning to that fact. Give me a Metagross with mash, zen, ice punch, hammer arm, tpunch, grass knot, bullet punch, and hp fire over 4MSS any day.
I'd also personally argue that steel/psychic does not make for "very good" neutral coverage. Being resisted by an entire type (steel) is not really ideal, especially when steel is an excellent defensive typing.
That being said, I'd have to pinpoint Mega Metagross' excellent offensive capabilities solely to Tough Claws, which is honestly a
retardedly really good ability. It's a life orb, with no life orb recoil, on a mon with essentially the same Attack stat as Haxorus, but with better speed (nay, excellent speed, in spite of the speed creep), better defensive typing, better spammable stabs ... and has priority. This tough claws boost is why Mega Metagross' coverage options still hurt so much, and why its STABs hurt even more - sometimes in spite of resistances.
I find it rather similar to Lucario (like Reverb) and Greninja in the sense that I don't really try to counter it anymore on my offensive or sometimes even balance teams, I just build to make a lot of my team able to dish out a lot of damage to it before it megas and check it. I don't think it's actually nearly as bad as either, but basically I understand where people are coming from.
Also I know people are trying to find counters for it; I thought I'd just mention one (sorta): Physically defensive rocky helmet Cresselia with twave. It's not really a counter in the sense that it won't reliably win one on one (meteor mash attack boost), but it has some counterish qualities in the sense that it can switch in on it and "take care of it," and if you need to preserve cresselia, you might be able to switch out into something that can take a hit from metagross and be able to kill it with it's speed drop since it's paralyzed. I remember this set starting to gain some popularity on ladder as people's catch-all "new mega" check when Mega Mence was tearing up OU - because it also checked Mence, Metagross, Lopunny, Gallade, Sceptile, etc. Anyway, it does many problems, like getting owned by pretty much any mon with substitute, being toxic fodder, getting trapped easily, not really doing much besides not dying, etc. but it does exist, although somewhat niche, and does "counter" Gross for the most part (i.e. provided they don't get like an atk boost meteor mash crit, and you better pray they don't have a cleric). Due to Cress' passiveness though it has to sacrifice lefties for rocky helmet, or else it'll lose to metagross in the long run over the course of the battle. Anyway, people seem to have forgotten this set exists since Mega Mence got banned, but it's still a fairly reliable answer to Metagross.
But yeah, for the most part Gross doesn't really have many counters (skarm with counter, scizor, Cress sorta, porygon2 sorta - loses in the long run, alomomola sorta, maybe a couple others).
