There are some interesting tricks you can pull with units that only have one attack capability.
I'm going to give an extreme example just to highlight how powerful this can be.
attacker has a devourer and a queen.
defender has a seige tank and two battlecruisers.
attacker sets the seige tank as the front line unit then plays improved flyer attack and spawn broodling.
the devourer can't hit the frontline seigetank but its power exceeds the tanks health so the supporting battle cruiser is destroyed triggering flying splash. spawn broodling kills the tank, and the flying splash is applied to the second battle cruiser.
In this example the zerg player had no possibility of loosing any units and the only way for the terran to save some of his units was for the tank to have 10 health.
There are many different combinations of this that can be done. For instance a reaver and a high templar could take down a corsair and two reavers by setting the corsair as the front line unit. The reavers attack exceeds the health value of the corsair killing one supporing reaver, using ground splash on the other reaver and ground/air splash from psy storm on the corsair. Again with no risk of losing anything.
There are a bunch more combinations but they all involve using an air only against a ground only in the front line(or vise versa) and then using an assist unit to kill whatever was in the front line.
spawn broodling/psy storm/plague/mind control/irradiate can all accomplish this.
You can defend against this by getting the defense module but you have to be aware of it first.