The impaler is functionally identical to the lurker with its ability to attack only while burrowed. However its attack is quite different, as the impaler is only able to launch a single tentacle to an enemy that does massive damage. It has 11 range, and deals 20 damage to a single target, 45 to armored.
Impalers are better offensively compared to lurkers, due to having longer range and stronger damage versus armored targets (which includes any buildings). However, their lack of splash attacks means they can't fend off multiple foes nearly as easily.
Impalers also require a clear ground route to their targets to attack them. If there is a chasm separating the impaler from its target, the impaler cannot attack it.
Impalers originally had an ability named "spine lash" that dealt damage to units and stunned them for three seconds. Associated upgrades increased the range of spine lash by 50%, the damage it inflicted by 50%, and the stun duration by one second, to a total of four seconds.