The persistent and shared EFS volume was the missing piece for running WordPress without frustrating patches.
And CloudFormation is the AWS best practice.
Convox has a few serious WordPress installs. The only difference is that it uses ECS so those EC2 instances can be utilized for more apps.
Also you can hack on the WordPress site locally with Docker via 'convox start'
https://convox.com/docs/wordpress/
Disclaimer: I work on Convox.
This allows you to sidestep a large amount of pain points with shared filesystems, and every ec2 server has local disk performance.
Latency on stat() - PHP is often a oop-monster these days with thousands of fs reads as it loads the code? (there are certainly ways to dodge that overhead though).
Have you seen the filesystem stall for any reason? Any caveats with concurrent writers (such as cache/ directories or generated images with deterministic paths etc).
You say without frustrating patches - are there any general limitations with using EFS?
Have you used self-managed NFS for the same purpose before? Any comparison with EFS?
This was on a workload much tougher than WordPress.
I haven't run my own NFS for comparison.
I recommend reading the NFS v4 spec. AWS has all the pieces to implement the protocol as well as anyone can.