Hi I have it installed on multiple servers:
2 frontend (expandable) with APF firewall, I use HTTPS on the loadbalancers with persistent connections
3 DB Galera Cluster using
www.severalnines.com behind a loadbalancer
Redis installed on single server (expandable) behind load balancer (expandable)
The one thing you do have to do is mirror the folders where, web version campaigns are placed, essentially I mirrored the frontend folder. I use Lsyncd, very fast and have yet to encounter an issue.
I'm currently looking at php-fpm, I've been reading about deploying a cluster which delivers PHPaaS this means you can have low end frontend servers and deploy servers configure specifically for PHP which in our case is sending load.
Haven't tried it yet but some bench marks I saw show a better performance for your buck and an easier setup to manage.