They're like a dagger you send in after back field stuff like weaker ranged units, support heroes and artillery crew. They're not the hammer/anvil the dryads are though and I believe the dryads make a better core unit, but the tree revenants are definitely helpful if you know when to throw them at what they want to hunt. I wouldn't throw them in alone and expect much to happen, the entire sylvaneth army is fast now so wait until you're got stuff near their target to support them or another assault somewhere and then strike with them that way they have more to deal with than just the tree revenants because if it's just them they'll likely be gone before they can really put on much pressure.
I also like to use them as objective grabbers.
The guy with the flute should always die last because if he's left alone you can teleport him out sometimes to some objective you guys might not be paying attention to. As for the weapon I like the extra hits better than the glaive.