Yellow is okay, Sigurd seems to be the only viable MC, Deirdre maybe. Elto/Cuan/Finn are decent but arent great. Yellow has some really unique card effects to deal with bonds, which is great.
Xander hilariously synergizes more with the old sibling promoted cards, because for the majority, the new ones are meh compared to the old ones or conflict with him turning off/on the dragon vein switch.
All of the siblings cause orbs to be flipped up, so they conflict with xander, other decks would be able to make better use of them (that dont use dragon vein, such as CorrinFWhite).
Camilla is alright, but you'll almost never flip 3 for the area destruction, but the 1 cost destruction every turn if there's an orb up is serviceable. Also, she's 30 support, so theres no real disadvantage to running her.
Elise can double attack, which is actually okay, and her supplementary effect is pretty good.
Leo is unfortunately meh, having to tap him to use his effect kills his potential, but i suppose a plain destruction effect is fine (coupled with his banning for 1 turn).
The ultimate problem is that by the time you get to use these dragon vein units, you'll have 0-1 orbs left to work with for flipping, and there's only 2 cards so far than can flip an orb back down to turn off effects (forrest and dead elise, which you would want for xander/camilla possibly), and since you have such a small resource pool, not many dragon veins can be used all the time.