Guardia Alta is definitely underrated when it comes to sword and buckler. You can attack the head using any line you want, even straight up through the chin. If you want to attack the legs, simply attack the head first and then use a molinello or two to reach the lower targets.
Were I studying single sword I wouldn’t be so confident. It seems to be that this guard is only safe because it lends itself to active use of the buckler.