I kinda disagree about Mao and Sej having poor 2v2 potential. They do decent damage (as long as they have mana) and have great utility to complement this, their burst damage is lacking, but you are able to defend yourself in 1v1 with enemy jungler if he tries to gank you.
I think they are not played that much because of mana issues and mentality of "blue buff comes to mid lane", and their weak top lane ganks. Bottom lane ganks tho are quite strong, i mainly play Twitch right now, but i used to play (and play if i get jungle role) Mao and Sej in jungle a lot, when i see opportunity to gank on bottom line, it's always double kill+tower. Rest of the time i can threaten enemy mid laner, counterjungle (stealing farm, not trying to 1v1 enemy) and try to get my health+mana items as fast as possible. Late game Mao and Sej are huge pain in the ass, they are very hard to peel and very hard to kill, while they do lots of damage and CC.
And little more about Mao, he can deal burst damage, but his main problem is mana, even with blue buff you will run out of your resource, i think this is main reason why he is not played right now. His knockups are great in fights, because they stop enemy from moving and slow him afterwards, and i think people underestimate his ultimate, yes it doesn't deal much damage, but its damage reduction what makes it good, combined with his passive he is very hard to put down.
But again, mana problems and long cooldowns on damage skills early game is what makes them weak in comparison to popular picks.
I don't know when was the last time when i've seen someone trying to 1v1 me in jungle on Sej or Mao, but last time it was KZ and i outdamaged him with my Q and red on Mao, i've had my passive proc'd 3 times and it was level 3. But i've been shut down by stealing my buffs via 3 ppl ambushes, it happened like, two times for my whole LoL experience, but i remember them both, it was awful and i couln't get back up