I agree with @samidodamage
on relying more on archers/rangers than dryads in a lot of situations, esp with mages.
Dryads do have higher health, but they also have smaller squads, almost half the number of an archer squad. My Dryads are only at Lvl 2 so they didn't get their poison darts yet, and buffs/debuffs are critical when facing 3 star enemies that all have them. Dryads also go last out of all the light range for humans, elves, and enemies. This makes them receivers of free pot shots on opening round instead of the ones dishing them out. In the higher provinces when I am grossly outnumbered, taking a pot shot might weaken them enough to render them pretty useless for the rest of the battle. In general, my Archers can take more hits, which is critical when every hit is pretty lethal in the higher provinces.
I also wouldn’t take them to kill mages. They can’t kill them in one shot like my Rangers and Archers even with fed Fire Phoenix and ELR. This is true in the mid provinces and in the upper provinces. Sometimes you only get one shot in upper provinces. Using archers/rangers also mean I've likely killed most of the mages on opening round so there aren't many left for dryads to use their defensive bonus against them. Any limping mages still around, I'd rather have them hit a ranger that also has mage defensive bonus and strikeback to finish them off.
I like Dryads in Heavy Melee+Light Range enemy combos, but I line them up across the HM due to initiative mentioned earlier. Mainly, I would very much love it if my Dryads can absorb as many hits as cursed Mist Walkers though! They don't perform nearly as well as their evil twin counterparts. Then again, nobody does