Note: I am talking about an atheist society - which has clear and discernible characteristics; and I am not talking about every single individual self-described atheist - atheists of course vary extremely widely.
As do Christians; and in fact most modern Christians are indistinguishable form atheists expect in terms of a few superficial 'lifestyle' features.
So what advantages are there to being an atheist society?
Well, let us first consider the dis-advantages.
These relate to the transcendental 'Goods' of Virtue, Beauty and Truth:
1. Virtue. Atheist societies are very recent - only abut a century old; but already include most of the most evil societies in the history of the world - the Soviet Union, Mao's China, Germany under National Socialism, the communist regimes of Pol Pot and current North Korea.
So atheism is at best a big disadvantage if you want a virtuous society; and quite possibly makes a virtuous society impossible.
(Of course, this is obscured because atheist societies are perfectly free to redefine evil as good - as in all the above examples.)
2. Beauty. From the industrial revolution onwards, the rise of secularism has - on the whole - correlated with the decline of beauty and the rise of ugliness. Any backlash against this decline in the beautiful has tended to be religious (if not necessarily Christian).
(Again, this is obscured since atheism is free to redefine the ugly as beautiful; so the arts have become more and more purposefully ugly, while claiming that this deliberately evoked disgust, boredom and alienation is actually a higher and more refined type of beauty.)
3. Truth. The most fully atheist societies are the least honest; since they have no reason for being honest about anything at any time - except expediency, and it is often expedient to lie.
The totalitarian atheist dictatorships are therefore a bye word for systematic falsehood; and modern secular Western societies have made giant strides in the same direction in the past half century.
We could also say that atheist societies are more miserable than Christian societies; and almost certainly atheists are on average much less-happy than devout Christians.
So, in conclusion: atheist societies are - on the whole - lying, ugly, evil and miserable.
So! What are their advantages?
Well, the main advantage of secularism - and this is an advantage which overwhelms all disadvantages for increasing numbers of people; is that in an atheist society there is no compelling reason not to do whatever it is you want to do, and to do it here and now; if you get the opportunity and can get away with it.
And there we have it - Ladies and Gentlemen!
The trump card advantage for atheist societies over all other previous and possible societies.