If you've ever put couatls in an altered melee map, you realize you soon have to nerf them because they're cheap (2 food, wow), but deal a huge load of damage fairly quickly. Actually, couatls are the only counter to red dragons in the Alliance mission where you have Illidan closing the portals in Draenor.
Dryads' Slow Poison may slow their attacks a bit, but they're unarmored and computer AIs tend to focus the one with lowest HP. Whilst you're spending more and more gold training them, computers are getting couatls for free.
On top of this, snap dragons will also deal huge chunks of damage to dryads, so you either have them sitting in the back, waiting for the couatl wave, or risk losing additional units fighting other compositions.
Plus, if you're using dryads' Abolish Magic, it'll be up to 4 times more costly to replenish life at the moon well, depending on the dryad:siren ratio.
Resuming, dryads are likely not cost worthy versus couatls. A good lot of ancient protectors will work much better: they're fortified, deal huge piercing damage to the flyers' light armor and you'll lose a maximum of 1 every wave, depending on how many you've placed. Things get complicated in TFT NE mission 2 because, sometimes, couatl waves will come escorted by mur'gul reavers, who can soak up a lot of hits from ancient protectors before dying while dealing some moderate damage to the building on their own. In this case, you may lose more than just 1 ancient protector.
Then again, they could work if you have a good lot of them not using Abolish Magic and micro them carefully (or perhaps by using moon wells' replenishment in auto-cast). Focus fires though...