The following section contains information from a previous version of StarCraft II: Heart of the Swarm which is no longer valid.
Dustin Browder had expressed concerns about the balance of the widow mine, namely its cost effectiveness and how it sacrifices itself to destroy the enemy. The optimum price point may be difficult to find in his mind.
The widow mine burrowed and waited for enemy units to approach. It then emerged, attaching itself to a unit. It exploded immediately. The blast did not damage other widow mines. Widow mines were better suited to taking out large, powerful units as opposed to masses of weaker ones, at least in regards to cost effectiveness.