Theists: What is the most compelling argument you have heard for Atheism?

[SteveII]

Amending your sentence to: " Also, if fine-tuning is uncertain on a universal scale, then would it be premature and somewhat short-sighted to apply this way of thinking to trillions of universes? [3]

[1] Fine-tuning is not uncertain. It is the case that our universe has a very precise set of values that if they were different, the universe would not exist (in any functional sense). Not only that, the ratios between the constants have to be precise. 

[2]All but a few of the trillions of universes would not be a place with any potential for complex structure, let alone life. Unless of course you engage in metaphysical speculation that the multiverse somehow it tuned to turn out pre-tuned universes--which just begs the question why is the multiverse fine-tuned.

Regarding [1],  if the precise set of values were different, then is it more accurate to say that the universe would not exist as humanity currently understands it? Also, does fine-tuning necessarily imply causal intelligence/agency? Is it possible that universes can fine-tune themselves?

Regarding [2], is it possible that there could be trillions of universes with complex structures and forms of life that are completely beyond humanity's understanding of complex structures and forms of life? 

Thank you for your reply, SteveII

1. A different set of values would produce a different universe (or none at all). Fine-tuning does not imply intelligence agency--but it does require an explanation. No, universes cannot fine-tune themselves--the initial conditions must be such that something can form. 
2. Yes, it is possible. It is not probable with our set of physical laws (which are different than the constants).