How reliable is Find My iPhone "Erase My Mac" feature?
This macobserver.com article details the procedure that is set into action when you click that big red 'Wipe Mac' button, and how that data could be recovered.

In conclusion...


  
  A remote lock state limits boot options on the Mac, but can still be circumvented by pulling the drive and mounting it on another computer.
  A remote wipe wipes all internal drives and, once complete, leaves access only to the Recovery Partition.
  


This Apple knowledgebase article also mentions that


  If the device is online, the remote erase begins within seconds (if the device is a Mac, it also restarts before it’s erased), and a confirmation email is sent to your Apple ID email address (the one you use with iCloud).