Yeah, I expect if the Second Secret was widespread, then Necromancer conflicts would often come down to one getting the drop on the other, take out their army with the Secret, then move in with their own.
I am confident that if the second secret was widespread, then necromancers and anyone with dhar construct armies would adapt to have them stand far enough apart, that you would not get large chain reactions anymore.
Would this require innovation in tactics/army composition etc? Yes. Is it better than your army being utterly worthless as soon as a half-competent spell caster stands close enough? Also yes.
My big wonder is how the Skaven never figured out the second secret of Dhar during all the time fighting Van Hal and/or others with undead armies. If the second Secret is an "I win," button against a necromantic army, (As long as you do not care about the Dhar aftermath, which the Skaven would not) and as long as someone on your side has enough understanding of Dhar to know the second secret, then you can use it to triumph over any pesky undead stopping you from harvesting warpstone.
My interpretation as to a likely reason this did not happen, is that once a necromancer is optimizing to stop the Second Secret taking out their army, provided they are competent or better at tactics, then it stops being an "I win," button.
Thus, it is only so effective now, because so few know about it?
Except, that doesn't fit with Vlad's army being taken out by the GT. Hmmm. Would he have potentially become stupidly overconfident because he had not seen it deployed against him yet, so in order to overwhelm more quickly, he packed his army in together beyond what was (Second Secret)SS safe?
Why did the Skaven never use it to destroy Van Hal's armies? Surely they had knowledge of the Second Secret, right? Intimate knowledge of Warpstone and Dhar is needed for much of their engineering.