I have the same issue with Undead (more or less) since there are (a) loads of Critters and (b) no Humanoids. I just use the undead against anything that is not critter.. and also not aquatic. That means.... yes. Only flying. My mech takes all the beasts. My critter takes the critters. My undead is not extremely useful.
But yesterday I came across a Pandaren Monk in pvp. The monk was pretty confident, my god it's a hard hitter. But my infected squirrel hit just a little bit harder. And it healed. I barely won.