Yosemite won't boot in safe mode with Shift held down
Ok, I managed to boot to Safe mode using this: 


  Access the command line by either opening Terminal remotely, or by logging into the computer using SSH.
  
  Use the following Terminal command:
  
  sudo nvram boot-args="-x"
  
  If you want to start in Verbose mode as well, use sudo nvram boot-args="-x -v" instead.
  
  After using Safe Mode, use this Terminal command to return to a normal startup:
  
  sudo nvram boot-args=""


Yosemite is looking more worrisome by the hour though.