You're also talking like we have to make sure he's balanced in Ubers, being that uber pokemon hit harder. Ubers is a banlist first and a tier second.
I'm not trying to troll you, so lets get that out of the way first.
Now then, does ME shadow tag stop switches before they happen, or no? Because when you are sending him in to revenge kill something, its a bit necessary
for whatever you are trying to kill to still be there when you go to kill it.
That's the first thing I want to know, if it doesn't outprioritize switching then MG is a poor mans mega alakazam. Second, you say "Why would I send it in on something with EQ?" Well lets say you don't. You manage to kill something, and they send in something with EQ, or they see your gengar and send in something likely to resist it. (Lets say Aegislash). Are you going to give up that first turn to give them a chance to set-up, or are you going to risk the destiny bond in case they want to kill you outright? How do you know if its scarfed? etc etc. Mega Gengar wouldn't be any higher than LO Gengar because what it gains in slightly raised stats and shadow tag, it loses in its newly acquired weakness to one of the most common and powerful types and a lack of item, on top of taking up a mega slot that would be better spent on something like Mega Ala or Mega Dactyl, on TOP of having lesser stats than a similar pokemon whom is
If no one else, I'd nominate Mega Alakazam for Ubers before I nominate Mega Gengar for ubers. I can see myself tanking an attack from Mega Gengar and returning with a huge hit. I can't see myself outspeeding something that comes like... 3rd to Ninjask and Deoxys-S while having higher SP.A than most anything else AND having great type coverage to boot and a somewhat useful ability in Trace.
If you are sending in Gengar, it would have to be a post-mortem at best because it won't be taking any hits it doesn't outright resist on the spot. Then you choose to/not to mega evolve it. Whatever attack you used, you'd better hope it kills whatever you are hitting because that pokemon will chew through your lack of bulk for breakfast. But lets give the revenge kill scenario a go, lets say you just swapped into something like Bulk-Up Conkeldurr. That thing is pretty beefy, and to cover for levitate it tosses out like... Stone Edge. If your shadow ball/Focus Blast/Sludge bomb doesn't kill it outright, Gengar isn't living the hit. Mega Gengar won't either, honestly. But lets say you Mega Evolve and kill it. Or lets say you don't and by some grace of Arceus kill it.
Go, Alakazam/Scarfed Terrakion/Aerodactyl/etc. Lord forbid they bring a juggernaut like Alakazam because that's just the end of the road. They throw out ME Psychic, EQ, Pretty much anything. Destiny Bond isn't going to save you when you are the slower pokemon, and switching will basically remove the "I have the choice to mega evolve" option. Does Mega Gengar have a niche? Sure. You can swap him out as long as ME Tag doesn't prioritize, blah blah. But is he uber tier? I wouldn't think so, no, on account of his entire kit and idea only works if you happen to be on the faster side. He is, in all means, a glass cannon. He is basically a faster Boomburst specs Exploud. If you aren't the faster mon, you don't get to keep going on. Its plain and simple. Revenge killer? He gets his job done better by most of the set in ubers, and looking at how potent megakazam is turning up as, it'd be tough to run him since he is just outright outclassed.
Like there are tons of pokemon that, given the chance to use their quirks, they can be phenomenal. If you manage to pass a +4/+2 from a ninjask onto something like Guts Façade Ursaring, yeah you'd be hard pressed to find anything to tank that hit. If you opponent lacks anything to check a Mega Gengar, yeah you'll probably sweep for free. But the point is, is Mega Gengar broken/OP enough to be cast out into Ubers tier? I personally don't think so. After all, why run Mega Gengar when you can just run MegaZam? Faster, stronger, only slightly less physically tanky (I misread the stats, Gengar has 65/80/95, still not stellar) has decent type coverage, huge breaking power, a great ability, far less weaknesses, lots more play potential outside of that first evolution point...
I personally don't see Mega Gengar being any more threatening than conventional LO Gengar. Ubers should be reserved for things that are nearly impossible to check at that point in time. Will ME Gengar never make it to ubers? He might, but for now he's nowhere near the level of SalaDance, classic mewtwo, or the like. Theres too much time left to see what kind of counter play develops, and Mega Gengar may just end up becoming the new Spiritomb. Regarded so highly early in the games life, but then it turns out no weaknesses isn't as glamourous as one might've previously thought.
Just my opinion though, I'd be far more afraid of MegaZam than I could find myself being of MegaGar