It's funny, just today I invented/came across exactly the same reasoning that Ruffen did.
The first premise of the KCA "Everything that begins to exist has a cause" makes perfect empirical, observational, and hence intuitive sense. We just don't see things popping into existence from nothing. This is where the idea originated; from observation.
Unfortunately, science has since told us otherwise. Quantum Mechanics involves particles doing exactly this - popping into existence - EVERYWHERE in the universe, all the time. Even in empty space.
The response from Craig is to say "Well, empty space isn't nothing. Time and space are still there"
So the question arises, what on earth would Craig accept as "nothing"? Where in the universe can we observe this "nothing" - where can we get away from everything, even space-time? Answer: we can't.
So Craig's definition of "nothing" is relegated to, quite literally, outside the universe. What does that mean? It means that the first premise - in stark contrast to how intuitively, empirically obvious it first sounded - is now merely a metaphysical assertion. Completely and utterly unverifiable, unfalsifiable, unobservable, unempirical. So his premise is really "Something can't come from "nothing", where "nothing" only exists outside the universe".
To Naturalism-ists like me, and in my opinion, this yields this premise completely pointless because it cant be tested/falsified. It's like me claiming that there are invisible, undetectable, unobservable three-legged bananas running the universe, making it look like it runs on the laws of physics. Pointless! Unless, of course, I happen to be a three-legged banana worshiper, the equivalent of which William Lane Craig most certainly is.
I do have an inkling that because Craig etc. do not adhere to empiricism or (metaphysical) naturalism they will claim that the first premise "Everything that begins to exist has a cause" can be known through Universals or as a Metaphysical Truths or some such nonsense.
Which immediately makes you wonder: Since the first premise really always was just "Something can't come from nothing, where 'nothing' only exists outside the universe", why try to dress it in sheep's clothing as the very empirical and obvious-sounding statement "Everything that begins to exist has a cause"?
Answer: If you can find empirical facts that seem to back up your belief, jump on them. But when empirical facts disagree with you, deny them and retreat back into metaphysics. This all seems wantonly disingenuous, but is actually probably just the delusionary confirmation bias, though I doubt it's been applied to philosophy before.