I agree with Birds that
birds, I mean eagles
are far from beeing weak atm.
Flying Nazgul also can be devastating, in certain situations. If your opponent isn't ready for them and doesn't have any archers/ranged units, then they can deal a lot of damage.
You can kinda compare them with cavalry: Cav units wreck swords, archers and heroes, can outrun their own counter (pikes), but once they get chaught by pikes they die quickly. Flying Nazgul wreck everything that doesn't shoot up, can outrun (most) archer units, but when archers get to shoot at them they die quickly. The difference between the two is, that cav can also be killed by archers and swords, flying units can't get killed by melee units. This means that ranged units need to be a harder counter to flying units than pikes are to cav.
This beeing said, I think that both eagles as well as flying Nazgul are in a good spot right now, in terms of combat power.
Concerning dragons:
But Smaug with nearly impermeable skin shouldnt be, it should be nearly unstopable force.
Well, I agree with you that smaug - if we go by the lore - should be able to kill every army you can build in Edain with one breathfire. Same goes for most bases. The problem however is, that we are talking about a RTS game. And in a game, Smaug, if he should be buildable in a normal game, needs to have some counter.
Let's take a look at the dragons in the valilla ROTWK:
They were able to set ablaze armies as well as building, they were able to deal a lot of damage. But, and that's the thing, you were rather squishy. You could kill them with archers, with heroes and with pikes (if he ran into them). Also Drogoth, the dragon hero, was able to deal devastating damage, but he also died quickly vs archers.
Now this "interpretation" of dragons is very different of what we see in the Hobbit movies or read about in the hobbit or silmarillion books, but it worked in terms of gameplay.
If Smaug (or any other dragon) in this game would be a "nearly unstopable force", as you suggested, than he would need to cost something like 20 000 resources. This would mean you almost never ever get to see him in a competetive game - and if you do, it's game over immediately, because of how strong he is.
Lorewise this would make sense, ofc: If smaug decided to join any battle, this battle would most likely be over right away, but gameplay wise this would be trash^^
The best way I see to have a really strong and op smaug would be by making him go super ham once you got the ring: If a ring is in the game the players usually aren't looking for balanced gameplay anyways, so a super strong smaug wouldn't be too much of a problem here.
But in a normal game (without ringheroes) every faction should be able to counter a flying dragon, even if it doesn't really make sense in terms of lore.
Best regards,
Seleukos