Very viable. Add the casters (branchwhyches & brachwraiths, which are basically also almost dryads) and Durthu plus Alarielle (which would be awesome riding some Merwyrm, coming from the sea), and you have more or less a cookie-cutter tournament list.
Beware of the fact though, that there are rumours that a new battletome might be coming soon. And then, nobody knows what's viable and what isn't.