Astonishing Speed has the potential to trivialize any quest phase. I don't think many players would hold onto three resources for this, but if they do, you're in for a treat. It does have the Rohan trait restriction, but would you put it in your deck if it wasn't chock full of Rohan allies? One last, off-topic point, that's a lot of dust coming off the ground

A two cost non-unique ally with two willpower and a handy response. West Road Traveller is pretty much as good as it gets. It would be neat if you discarded it instead of trigger when it enters play to synergise with the Rohan mechanic, but that would push it over the top. The only downside is it's one HP.

Support of the Eagles is a powerful, neat card. At face value, it's a big boost to one of your heroes for pretty much the whole game. Dig a little deeper and you realize that with readying effects, it's bordering on OP. You've found a way to make Boromir a superhero without breaking the game. It's also a good way to keep hold of your Winged Guardians or Vassals if the Windlord without losing them. Along with a pumped up EotMM, I wouldn't want to be an enemy.

Eagles of the Misty Mountains is the driving force of the eagle deck. While overcosted at first, there are so many ways eagles can leave play that these guys will be beasts in a couple turns. Support of the Eagles adds to the madness. So if you have four resources laying around and one of these in hand, you know what to do.

Dawn Take you All is not that good. It's good in theory, paying two resources to cancel up to four shadow effects, but it doesn't really work in practice. There's always the chance that the shadow cards don't have effects, or much more frequent, that more enemies will be engaged with one player than the others. There are just to many other better options.