Those bane combos do so much damage because Bane is popping all 3 of his venom tanks, wich is a VERY risky move, depending on what level you are playing at. At high levels, Bane would be unlikely to land such lengthy combos without being interrupted. Because of this, players will often only pop all three of his venom tanks if it's going to result in the win. Bane is fatigued after his venom runs out, and takes more damage/is slowed, so it's high risk high reward.
That being said, Bane can get nasty damage.