The card is slow is the problem. It's very effective at what it does, but it has 0 closing speed, and can be raced by everything. It was fine in the Stoneforge decks of past, since once you put some pants on it, it finished the game very quickly--but as is, it just takes too long to end the game.
I think young pyromancer is better, and it costs 1 less mana. TNN is good for holding the ground in the beginning of the game against shops, against outcome it's basically the same as a 3 mana 3/1, against survival it holds the ground until they start flying. That being said, it does kill planeswalkers and have a slow but steady clock. But at 3 mana, there's managorger hydra that is an unstoppable monster (in terms of P/T), that can be exiled or bounced, but in a control deck you can usually protect it.
I'm curious why you think TNN is a good defensive creature in the metagame.
Most decks in the format are focused on going wide, whether it is in the form of tokens (Dredge, Mentor, Pyromancer) or volume of threats (Ravager, Survival), or over the top of a vanilla 3/1 (PO, Oath, Combo). The good matchups for TNN are pretty few IMO and include Eldrazi, random BUG Fish decks, and Planeswalker based control decks, which would be about 10% of the current MTGO metagame.
I love TNN and I played him before Mentor was printed. Once Mentor was printed and we went to essentially all Blue decks playing the token kill or they play for a fast kill with Tinker Bot. The creature math is bad in Shops and Dredge and he does not do enough work fast enough.
In this new counter everything meta, TNN is blue and he would be contested on the stack. Unless you are playing Merfolk with Cavern of Souls this battle is not worth it for most decks.
@neo_altoid Would that matter though if you were playing a control deck? Also, I would imagine you would try to not play it alone. Could it serve the role as a harder to remove, but more expensive Delver in some tempo decks?
There is not a viable control deck in this metagame.