Gengar is like a sniper in the sense that it can target anything it wants to and eliminate it without question. Therefore, when played to Mega Gengar's strengths, I, as much as I never wanted this to be the case, think it's an unhealthy threat in the metagame and can slowly but surely easily pick off the opponent's team. My reasons:
1. Shadow Tag enables it to target almost any Pokemon it wants to and essentially eliminate it from the game.
This is the biggest advantage Gengar has over basically any other Pokemon. It can select almost any Pokemon at any period of time during the match and, given the opponent is weakened enough (this isn't difficult with entry hazards and the like), you basically get a free kill. This means that it can dispose of troublesome walls, such as Skarmory and Togekiss, for your sweepers to wreak havoc. It's not even just walls, either. See that weakened Tyranitar? See that Excadrill? That Starmie, Breloom, Garchomp, Greninja? They're gone. And it's that easy. The fact that Gengar can do that to a very decent effect is worrisome.
This means of support is invaluable to the many sweepers that are hard-walled by threats, too. Even if you're not using it for that support, just the fact that it can slowly but surely pick off Pokemon by Pokemon with that Shadow Tag and amazing Special Attack and Speed is superb. Speaking of which:
2. Base 170 Special Attack and base 130 Speed is phenomenal for a revenge killing trapper.
Even though it's slightly weaker than its Life Orb Gengar counterpart, the huge increase in Speed quite literally makes it the be-all-end-all of revenge killers. The threats that used to outpace it, such as Starmie, Greninja, Weavile, Alakazam, and Thundururs, are now outsped and all of them are roasted by its STAB attacks bar Focus Sash Alakazam. That base 170 Special Attack is nothing to scoff at either. Basically all of OU is either 2HKOed or OHKOed bar the specially defensive behemoths, most notably Chansey and Blissey. But wait, is Gengar actually "walled" by anything? It pretty much isn't because:
3. Destiny Bond and Taunt provide answers against many of Mega Gengar's checks and counters.
Destiny Bond/Taunt + 3 attacks is the Gengar set of choice currently. Each move allows it to accomplish various things. With Destiny Bond, no longer can it be revenge killed by Sucker Punch users, particularly Kangaskhan. They will be taken down with it the next turn if Gengar chooses to attack. Destiny Bond almost ensures Gengar gets at the bare minimum a total of 2 kills. It's first kill is from revenge killing something that is helplessly trapped, and kill two comes from Destiny Bonding the switch-in if it chooses to do so. Of course you can switch Gengar out, pick off something else later, and Destiny Bond then for 3, 4, or 5 kills. Who knows? In terms of Taunt, it turns Gengar to the epitome of stallbreaker. Not only can it obliterate physical walls such as Skarmory, opening up sweeps for your physical sweepers, it now has the ability to blow up Blissey, Chansey, and Togekiss, among other special walls, with Taunt. Now they can't paralyze you, Wish + Protect stall, or just heal up with Soft-Boiled or Roost. They have no choice but to succumb to Gengar's attacks which hits all of them super effectively. Defensive and stall teams are crapped on. Offensive teams are still crapped on. All team styles are crapped on.
4. Great number of switch-in opportunities.
Yes, Gengar is frail. Yes, Gengar loses an immunity to Ground. But that doesn't mean it can't switch in. It's actually really easy to bring Gengar unscathed. U-turn and Volt Switch cores are riddled in the metagame, being effectively used by Pokemon such as Talonflame, Rotom-W, Scizor, and Greninja. All of these Pokemon force switches like mad, meaning Gengar can often come in without taking any damage if the opponent switches in something it can take out, which can very well be the case. Talonflame lures in Tyranitar, Focus Blast that to death. Scizor lures in Gliscor, Shadow Ball that to death. It doesn't stop there. There's also "double switching" which is what also makes Gengar that much scarier. If your opponent predicts you to switch to something, in comes the switch to Gengar. Finally, Ghost/Poison is not a bad typing. Immunities to Normal and Fighting, which are pretty common actually, is great.
All in all, Mega Gengar has all of the tools it needs to be the #1 revenge killing trapper in the whole game, easily dismantling ANY team with its Shadow Tag, base 170 Special Attack and 130 Speed, perfect type coverage moves, and Destiny Bond/Taunt. This isn't even considering Perish Song or Pain Split, which will further increase Gengar's revenge killing potential. Ban Gengarite.