Yes, more variables to deal with, that’s a good point. And it would help if it isn’t stackable.
What I think the difficulty is in is making it both useful and not overpowered, and in a way that is not too strong in lower levels while not strong enough in higher levels. That mainly has to do with the DoT aspect more than anything.
In BW, often Science Vessels can dip in and out, often in between engagements, and that’s how they get the damage done. That kind of unit dancing plays out differently in SC2, where most abilities where that happens are immediately impactful (whereas DoT takes time by nature), and where Abduct exists which makes that kind of play very risky against Zerg particularly.
Also, because of pathing and easier unit management in SC2, DoT in SC2 is generally best against clumps of massable units where more effort needs to be made to split, which also reduces DPS of the units being split in an engagement.
Now, think about the most important targets of Steady Targeting. Brood Lords, Ultralisks, and Zerg spellcasters. These aren’t often in large, tight clumps, except maybe Brood Lords sometimes, and some have lots of HP. The fact that Transfuse exists means that if damage does not occur in bursts, it’s going to be easily negated by Queens, and that’s going to happen more in higher levels where Queen control is better than in lower levels.
So with those things in mind, how do you make Irradiate actually effective against the units it needs to be effective against at high levels, without making it too strong against other units or too strong at lower levels?