i) Irrelevant, there is very little power difference (LO has like 10% more power), and since Mega Gengar chooses what to kill, it's doing a lot more damage.i) Mega-Gengar deals less damage than LO Gengar
ii) Lack of priority moves hurt (CB scizor, Talonflame, Aqua Jet, Shadow Sneak, Sucker Punch)
iii) cannot make full use of ability
iv) EQ weakness
v) predictable moveset (when they see mega evo)
vi) definitely not ubers material (too easy to play around)
Gengarite should not be banned.
ii) Does a Base 130 Speed mon really need priority? Just switch out of priority.
iii) Focus Blast when you Mega to catch Tyranitar, or Sub when you Mega, and afterwards it can switch in and trap right away.
iv) If you keep your Mega Gengar on something with Earthquake you are very retarded.
v) If you think that Perish Song/Taunt/Disable/Substitute/Shadow Ball/Sludge Wave/Focus Blast/Dazzling Gleam/Thunderbolt/Hidden Power/Destiny Bond is predictable then you have been using Mew way too much.
vi) You don't play around Mega Gengar. Period.