Brimaz's effect triggers during every end step, not just your own, so if you have some form of sacrifice outlet, you can ensure it always triggers
Well, Doomskar is technically a five-mana spell
The cost reduction is fine but nothing special, and the protection ability is flimsy
Casting creature spells from your graveyard using their sneak abilities with the permission granted by Ninja Teen's first level 3 ability doesn't change when you can cast them
On top of that, if you end up milling over something like Champion of Wits for later, you start to create a lot of lasting card advantage as the game goes long
Because of this, they are not as valuable to players who actually want to add them to their decks