Why are shortcodes lowercase and with underscores?
To reduce the room for error. In the future we might extend functionality e.g. with attributes, but for now, this is the safest solution.

I don't see the option to add PHP snippets, why?
This option is only available for Super Users.

I can't edit certain shortcodes, why?
PHP snippet shortcodes can only be edited by the Super User.

My PHP shortcode is not replaced, why?
There is probably a bug in your PHP code. If it raises an error, it'll be replaced by NULL. The error may have been logged to the Logs page. Shortcodes are, at the moment, not replaced by NULL replacements. Please verify if your PHP code is correct. E.g. via http://sandbox.onlinephpfunctions.com. Make sure you echo a value, and that you don't use the <?php opening and ?> closing tags.

My site is totally bricked after I added a PHP snippet
Go to /dashboard/shortcodes and either 1) Disable the add-on via the Settings page, or 2) Disable the shortcode, 3) Change the PHP code of the shortcode.

Does this work with Full Page Caching?
It works with FPC, but after the page is put in the cache, PHP code will not be parsed again and the event won't be triggered again. So be careful when using event shortcodes or php shortcodes.