The Katateka explains why a boxing guard leaves the fighter open to midsection kicks (also high kicks above the chin) . However, the Karate guard is loo low against continous punching, also known as boxing!
Some of the Thais use a high guard protecting the top of the head, but leave their body wide open for front kicks and strikes to the body in general.
Which guard do you think is optimal for freestyle stand-up? MMA fighters get tagged no matter which guard but it's hard to hide behind those small gloves.