The tiering system is as fair as can be imo. Ubers is a banlist not a tier, however due to all the Pokémon that do get banned there is enough Pokémon to actually have a competitive metagame up there.So then Deoxys-N is rarely ever used then since it's banned to Ubers and doesn't do to well up there. In regards to Mega Gengar, I don't do wifi battles anyways so I would just use him in GBU battles and he'd do alright there so this in no way affects me but still, tho I don't wifi battle i want to see if MGengar does well up there incase if i ever do want to Wifi battle and do ubers....But right now i'll stick to using MGengar in GBU. Tho, yeah it seems alittle stupid that pokemon get banned to Ubers and then rarely see any use. it make me think the tiering system could be ajusted alittle, that's my opion tho.
As to all the people saying Mega Gengar wont be Uber is because they are either facing in component players or because a large number of people are trying to use him the same way they use Gengar. MGengar is an amazing trapper and is extremely Potent late game where it can start trapping and kill off weak Pokémon. MGengar is the mega I fear most when playing competent players as there gets to s stage in battle where as soon as they send him out they win.
I see to many people try to use him like regular Gengar, continually subbing which is pointless because the Pokémon cant switch out and is forced to keep attacking you. MGenagar should only be subbing on Pokémon that cant touch you or break you Sub to give you free rain to hit the next Poekmon hard. MGengar is best used as a trapper, trapping and KOing Pokémon to clear way for other members of your team to sweep. For example trapping and killing RotomW paving way for your Talonflame to wreck the opponents team.