I play a bit of Ubers, and honestly, I've got a really split opinion about this
I know from experience and use that Mega Gengar is a fucking broken piece of shit, and is extremely difficult to deal with when a good player is using it
Gengar has an extremely diverse movepool and hits extremely hard with Spa rivalling Kyurem-White, most notable examples are
>Shadow Ball
>Sludge Wave
>Thunder/tbolt
>Psychic
>Focus Blast
>Giga Drain / Energy Ball
>Dazzling Gleam
It is extremely difficult to check MGengar as you need to constantly be ahead of your opponent and one wrong move is all you need to lose a mon ie, "Should I Close Combat this Arceus, or is he going to switch to Gengar, but if I use EQ, he's going to SD because he can take a few EQs and sweep half of my team with ease and have Gengar finish it off"
It is extremely reliant on successful prediction, worse than Aegislash in OU, you cannot make the wrong play as it can cost you the entire game, and even if you can predict a Gengar switch in, you're fucked because of Taunt+Destiny Bond.
There are ways around Mega Gengar, using BP, U-turn, Volt Switch, Shed Shells (which Blissey is forced to run, because you can't risk your wall being ruined with no way to beat Gengar)
As I've previously mentioned, it is extremely difficult to check MGengar, because of its diverse movepool, you cannot tell if it's supposed to be a check for Xerneas, Yveltal, Arceus, Kyogre, Mewtwo etc UNTIL you have lost a mon against it, it may be running HP Fire to deal with Scizors that check Xerneas, it may be running Thunder to deal with Kyogre, Lugia, Ho-Oh, it may be running Shadow Ball to check Mewtwo
cont.
