I personally believe it to be unwise to come to a conclusion until Pokebank comes out simply because there are Pokemon that will trouble Mega Kangaskhan that are not available yet, namely Terrakion, Landorus-T, and Keldeo.
Mega Kangaskan dares not challenge Terrakion head on once its in; I really think that it'll have a hard time surviving a LO or CB boosted Close Combat while being outran. The only thing it can do once Terrakion is in is Sucker Punch, which would give it +2 attack and put your team at serious risk of being swept.
Keldeo is in a similar boat, of course not getting +2 to its main attacking moves, but still threatening Kangaskan with a powerful Secret Sword. We've seen how hard Specs Keldeo hits; easily OHKOing naturally bulky Pokemon such as Black Kyurem who sports better bulk than Mega Kangaskhan.
In present play faster and Prankster Pokemon carrying Will-O-Wisp are, in my experience, proving to be a serious pain the tail for Mega Kangaskhan with Prankster Sableye, Gengar, and Delphox being examples. Gengar gets the most credit here, as he is immune to Mega Kangaskhan's Earthquake, Return, Double-Edge, and Power-Up Punch making his only real vulnerability to Sucker Punch(or the rare Crunch, but I believe Sucker Punch to be the superior option so as to not be complete prey to Choice Scarf users) which basically guarantees him to be a safe switch in. I have seen very few Gengars running Will-O-Wisp since he gained access to it, but I have always believed it to be a very solid option for him. One can argue that it is only one Pokemon that has a safe switch in to Kangaskhan, but Gengar is and always has been used by loads of people from the days of R/B/Y and he can fit on to basically any team effectively especially now that he has access to Dazzling Gleam to give him a reliable option over HP Ice to deal with Dragon types, he has gained even more versatility and unpredictability
Although one issue I foresee is that Pokebank will allow Mega Kangaskhan access to Seismic Toss, previously only available from FR/LG tutors. This would allow an EV spread of nothing but bulk and allow him to still fire off a Seismic toss for 200 damage, guaranteeing 2HKOes on the vast majority of the tier with only the beefiest Pokemon surviving with Black Kyurem with some HP investment being a notable example that would survive. Other notable moves that it will gain access to being Aqua Tail, elemental punches, Drain Punch, and Low Kick.
In the end, my take on the matter is to wait until extensive testing in Pokebank OU has been conducted though I personally believe him to be completely manageable due to his low speed tier and having to rely on Sucker Punch for a priority move when there are faster users of Will-O-Wisp running around, as well as Prankster users.
I realize that probably most, if not all, of what I have said has been discussed/posted already, but I merely wanted to give my analysis to it :)