My first thought is that this has the same ambiguity issue as Ferocity. It’s not clear if it’s immediately affecting the units currently applicable to it, or more abstractly applying to the turn / particular step in the turn (in this case, the readying step).
My personal feeling is the ambiguous wording reads as an intent to affect all units (not just the ones it would seem to apply to, otherwise it would say something like “the units you have as you cast this” instead of “your units” or “opposing units”)
We’ve ruled on Ferocity in the past that it is only the units you have as you cast it. I’m generally inclined to think the same applies here.
This post by Armed Pirate is the only thing that gives me pause. There’s no “get” here as in Ferocity. It talks specifically about “during their next upkeep”. Nothing about now, as I cast this. Still it’s ambiguous, so we have to interpret, “opposing units that are exhausted” WHEN? The ones exhausted right now, as I cast Ready or Not? Or the ones exhausted as you do the check during next upkeep?
I want to hear from @Dreamfire and @charnel_mouse here as well. It makes a big difference to the game at hand and we really haven’t discussed it in the rules thread (I’m going to post a link to this there now)