I've been using Apache behind Nginx for years, and not once has it gone wrong. HTTP reverse proxying is not something overly complicated that can go wrong often.

How it is best of both worlds? Nginx simply does not have all the features Apache have. Take a look at the .htaccess rules that Wordpress and WP-Supercache needs. It's much easier to to run Wordpress on Apache and put that behind Nginx than to figure out how to rewrite the rules to Nginx form (or worse, update the rules when they change). There's also a ton of Apache modules that have no Nginx equivalent, or no easy Nginx equivalent.

Why I use PHP? Because some apps don't have good Ruby or Python equivalents. After all these years I still haven't found anything better than Wordpress and phpMyAdmin. If it's my own code, sure, I'll write Ruby, but I'm talking about apps.

Putting Nginx before Apache allows it to protect Apache from slow clients because Nginx buffers requests and responses.