They are pretty freaking bad. They will never compete with other hunter decks or anything in the higher ranks. They seem alright when you pull them off, but it can only go so far.
The Toxmonger is more of a tech card, works so well combined with Halazzi! Ecspecially with these huge minion boards nowadays. Dinomancy is also handy to make beasts survive longer, until they're out of reach.
Main thing, both don't really need more than one in a deck.
In my opinion dinomancy is not very good. It is in opposition to what most aggro / midrange hunters want to do because it gets rid of a useful late game hero power. I also don't think dinomancy would be good in control hunter because if you don't have consistent board presence it is useless.
It's not that either are necessarily bad, it's just....what better cards are you not running in order to run these cards instead, and why are you running a sub par decklist?
Now that THAT'S out of the way, Dinomancy is bad because your standard Hero Power is good at unconditionally killing the opponent; this replaces it with a conditional and less good Hero Power. Toxmonger is a value card that requires setup, and Hunter is typically a tempo class. You know what's good in competitive decks? In tempo-centric classes/decks, cards that are good on their own typically make the cut, and cards that require synergy or setup don't. Deathrattle Hunter gets away with it because the value of Play Dead, Carnivorous Cube, and Terrorscale Stalker is insane. Once again, Hunter is a tempo class and Toxmonger is a conditional value card.
Rexxar's HP is just so much better. Even if he happens to be towards the bottom of your deck, it's not worth running two weak cards that are completely dead once you draw Rexxar. It's just not worth it, especially when Zul'jin is another more-powerful card that replaces your DK
if you're at the point where you can hero power and play your turn in hunter, you're either losing or better off just KILLING them with your starter power. Not to mention you have to spend 2 mana and a card to activate dinomancy
"I have seen worlds bathed in the Makers' flames, their denizens fading without as much as a whimper. Entire planetary systems born and razed in the time that it takes your mortal hearts to beat once. Yet all throughout, my own heart devoid of emotion... of empathy. I. Have. Felt. Nothing. A million-million lives wasted. Had they all held within them your tenacity? Had they all loved life as you do?"