Maybe I haven’t found my groove with him, but I agree with the OP. I’m not enjoying Ajani at all. What I want to do with these colors is jam a bunch of supports and spells together, and the deck restrictions prevent this. Also, he is the only green Planeswalker who generates 5 mana on a 3-gem green match. He gets 7 on white, but how do you flood the board with white? And if you do, you risk popping the few supports you can put into the deck.
All of his abilities generate energy, but there isn’t much in white and green I want to do with energy. There are some good colorless supports which deal with energy, but the bonkers ones are mythics I don’t have and the decent ones have to compete with every other green / white support for the meager 4 deckslots.
By comparison, Ajani I generates 6 on a white match, 5 on a green match, 5 on a red match, 4 on a blue match, 3 on a black match. Compare with 7,5,3,3,3 for Ajani II. His abilities can keep the opposition at bay while powering up the Oath, or one of the Oath creatures, or one of the combo cards that can cheat Oath into play.
Plus, Ajani I can put 9 critters, 6 spells or 5 supports into a deck, vs Ajani II’s 8,5,4.
Even without Oath or Pig, any ridiculous thing I want to do with white or colorless, I can do more easily with Ajani I. Any ridiculous thing I want to do with green usually involves turning all the gems green and profiting from that, but Ajani II has the weakest mana production of any green walker, profits the least from turning all the gems green, and has fewer deck slots available for the type of cards which turn, or profit from turning, all the gems green.
If I want to do stupid things with energy, I’d rather do it in red-white than green-white, but maybe that’s a card availability issue. I suspect not, though.
And the first ability? Gain 9 life? From the walker with the highest starting total in history? It’s really only relevant if you’re getting beaten down by your opponent because you don’t have enough slots in your deck for the spells and supports which prevent the beatdown, and you don’t have enough mana to power up the removal.
At least, that’s been my experience so far. Your mileage may vary.