I'll post the two basic yes/no answers for folks to vote on.
Well, you deleted those answers (with "no" at +5/-0 and "yes" at +0/-4), so here's my take on it.
Used for questions seeking plausible descriptions of factual scientific failures.
Here already, the tag seems awfully similar to perhaps a combination of reality-check and science-based (which themselves are somewhat problematic, being meta tags).
Explaining things is something we pretty much do all day long.
Explaining things that can't be explained by science is more or less the bread and butter of this site. It's how we end up with popular tags being for example xenobiology or post-apocalypse or mythical-creatures or time-travel or super-powers or terraforming or faster-than-light or interspecies-relations or merfolk or teleportation or shapeshifters or... well, you get the idea. Even though they (and many other things that get discussed regularly on Worldbuilding SE) cannot be explained by science in our world, that's no reason why explanations cannot be internally logically consistent. We also established very early on that unless the question specifically allows for magic, magic can't be used to answer that question.
A proposed tag should make sense in a sentence on the form "I'm an expert in ...". Those I listed above, and many others that see significant use, seem to pass that test pretty well. "I'm an expert in time travel" isn't an unreasonable thing to say, even though one could argue that time travel is impossible according to our understanding of our universe (which is a different issue). "I'm an expert in technobabble" sounds rather less useful.
What's been done here seems to me to be more or less to add a tag that could apply to just about every question on the site. That adds nothing useful for categorization of questions, and is another slot used out of the five tags one can add to a question.
My vote is for no, it's not a good tag.