The implementation is different, but both are basically anti-biological spells meant for taking out high value Zerg targets. That is where they overlap.
I would argue that Irradiate is easier to balance for a couple reasons:
- Stacking can be removed just like with Parasitic Bomb, so that casting multiple Irradiates in an area does not increase the rate that it deals damage. That is a problem that Snipe, Steady-Targeting, and most variations of Irradiate all share. This change would likely discourage massing the unit that has the ability beyond a certain point.
- Irradiate also has more variables to adjust to find a balance point for it.
- The fact that Irradiate deals damage over time means there is more time to react to either mitigate AOE damage or transfuse the target.
- The last reason doesn’t have to do with the spell itself, but the spellcaster. The Ghost doubles as a decent combat unit, so it is trickier to tune its spells than it was for the Science Vessel. The Raven just suffered from having a bad kit or problematic kit for most of its existence.
While Parasitic Bomb operates like Irradiate (without the stacking damage or hitting ground), it also covers 3x the radius (9x the total area) and persists after the death of the target. The DPS of the spells is the same though.
Fundamentally, spells like Snipe, Irradiate, and Steady-Targeting (all with some way to stack damage and kill things quickly) encourage massing the casters to abuse those mechanics. The same thing was possible with Science Vessels and Irradiate, although this was more difficult to control in the SC1 engine.
Something always has to be done with casters that have spells like that to discourage players from just massing the unit.